soldthehouse Posted August 16, 2009 Report Share Posted August 16, 2009 What are the pros and cons of a pointed swim as opposed to a flat end plate where the stern tube exits the hull, is cavitation altered considerably or is it just paper theoretics and pub talk gone mad. Link to comment Share on other sites More sharing options...
blackrose Posted August 17, 2009 Report Share Posted August 17, 2009 (edited) What are the pros and cons of a pointed swim as opposed to a flat end plate where the stern tube exits the hull, is cavitation altered considerably or is it just paper theoretics and pub talk gone mad. I'm not quite sure what you mean and I've never heard anyone discussing such matters in a pub! The main difference I've noticed in the underwater design of bow types are those in which the sides are brought together all the way down to the flat baseplate, as opposed to those like mine in which the baseplate is curved up at the bow to meet the sides. I think the latter type saves some metal so it is favoured by some builders, while the former type is supposed to cut through the water more efficiently rather than planing over it. Personally at the speeds we travel I doubt it makes much difference. Edited August 17, 2009 by blackrose Link to comment Share on other sites More sharing options...
mayalld Posted August 17, 2009 Report Share Posted August 17, 2009 I'm not quite sure what you mean and I've never heard anyone discussing such matters in a pub! The main difference I've noticed in the underwater design of bow types are those in which the sides are brought together all the way down to the flat baseplate, as opposed to those like mine in which the baseplate is curved up at the bow to meet the sides. I think the latter type saves some metal so it is favoured by some builders, while the former type is supposed to cut through the water more efficiently rather than planing over it. Personally at the speeds we travel I doubt it makes much difference. The question related to the design of the STERN, not the bow. Certainly previous discussions have suggested that there IS a differenc in performance between the rear swim meeting like a blade, and meeting with a 2" flat. Link to comment Share on other sites More sharing options...
blackrose Posted August 17, 2009 Report Share Posted August 17, 2009 (edited) The question related to the design of the STERN, not the bow. Certainly previous discussions have suggested that there IS a differenc in performance between the rear swim meeting like a blade, and meeting with a 2" flat. Ah, that'll teach me not to respond to posts on my mobile phone, I thought it said stem post! However my original point about minor differences in hull design making little difference at the speeds we travel still stands. Canal boats are not high performance craft. Edited August 17, 2009 by blackrose Link to comment Share on other sites More sharing options...
Albion Posted August 17, 2009 Report Share Posted August 17, 2009 The question related to the design of the STERN, not the bow. Certainly previous discussions have suggested that there IS a differenc in performance between the rear swim meeting like a blade, and meeting with a 2" flat. It is clearly beneficial to deliver the water to the prop as smoothly as possible and as much as possible. A blunt-ended stern swim isn't going to do this as well as a pointed one as there will be some turbulence when the water leaves the tip. However, how much difference this makes on a narrow boat is difficult to quantify without scientific tests I reckon. Suffice it to say that the more pointed the swim the better IMO. Roger Link to comment Share on other sites More sharing options...
Timleech Posted August 17, 2009 Report Share Posted August 17, 2009 It is clearly beneficial to deliver the water to the prop as smoothly as possible and as much as possible. A blunt-ended stern swim isn't going to do this as well as a pointed one as there will be some turbulence when the water leaves the tip. However, how much difference this makes on a narrow boat is difficult to quantify without scientific tests I reckon. Suffice it to say that the more pointed the swim the better IMO.Roger Agreed. With the blunt stern post, the distance between prop & stern post will become very significant. Tim Link to comment Share on other sites More sharing options...
RLWP Posted August 17, 2009 Report Share Posted August 17, 2009 Presumably, on the other hand, a blunt ended swim is easier to make and put the stern tube through? (awaits input from a boat builder) Richard Link to comment Share on other sites More sharing options...
furnessvale Posted August 17, 2009 Report Share Posted August 17, 2009 Presumably, on the other hand, a blunt ended swim is easier to make and put the stern tube through? (awaits input from a boat builder) Richard Perhaps boat builders could finish with a half round tube. The length of weld would be exactly the same and the finished job would look like a Woolwich boat at least. George ex nb Alton retired Link to comment Share on other sites More sharing options...
hughc Posted August 17, 2009 Report Share Posted August 17, 2009 Of course narrow boats are high performance craft. They are not fast in restricted channels though many well designed hulls will reach their maximum displacement speed in open water. Because narrow boats fit the channel so closely every small improvement in how they swim will make a noticeable difference. Many modern cheap clones swim like bricks because the design input was minimal. Bluff bowed and short swims on GUCC boats didn't affect their speed because the overall underwater shape was considered as a whole by qualified naval architectts and the subtle curvature aided water flow in narrow channnels. Flat ended sternposts create considerable turbulence which affects propeller efficiency to a large degree. Wake reduction plays an important part in the choice of propeller. If the prop. is in free water then the speed of the water into it is the speed of the boat. The more restricted and turbulent the flow the greater the difference between boat speed and propeller speed because water is being dragged along by friction. Warren gives a range of wake reduction from 0% in free water to 30% for a heavily masked prop. Performance is not related solely to speed and a boat that swims well leaving little wash is a joy to steer. Regards, HughC. Link to comment Share on other sites More sharing options...
soldthehouse Posted August 17, 2009 Author Report Share Posted August 17, 2009 I'm not quite sure what you mean and I've never heard anyone discussing such matters in a pub! The main difference I've noticed in the underwater design of bow types are those in which the sides are brought together all the way down to the flat baseplate, as opposed to those like mine in which the baseplate is curved up at the bow to meet the sides. I think the latter type saves some metal so it is favoured by some builders, while the former type is supposed to cut through the water more efficiently rather than planing over it. Personally at the speeds we travel I doubt it makes much difference. It is the stern post i am enquiring about not the bow Presumably, on the other hand, a blunt ended swim is easier to make and put the stern tube through? (awaits input from a boat builder) Richard I.m building the boat and I just thought it must be easier to fit the stern tube to a flat plate and wondered if turbulence would be a problem. I didnt personally think it would be but I knew that the many years of experience on here would come up with the pros and cons, thanks for your input Of course narrow boats are high performance craft. They are not fast in restricted channels though many well designed hulls will reach their maximum displacement speed in open water. Because narrow boats fit the channel so closely every small improvement in how they swim will make a noticeable difference. Many modern cheap clones swim like bricks because the design input was minimal. Bluff bowed and short swims on GUCC boats didn't affect their speed because the overall underwater shape was considered as a whole by qualified naval architectts and the subtle curvature aided water flow in narrow channnels. Flat ended sternposts create considerable turbulence which affects propeller efficiency to a large degree. Wake reduction plays an important part in the choice of propeller. If the prop. is in free water then the speed of the water into it is the speed of the boat. The more restricted and turbulent the flow the greater the difference between boat speed and propeller speed because water is being dragged along by friction. Warren gives a range of wake reduction from 0% in free water to 30% for a heavily masked prop. Performance is not related solely to speed and a boat that swims well leaving little wash is a joy to steer. Regards, HughC. It will have to be pointy then Link to comment Share on other sites More sharing options...
BEngo Posted August 18, 2009 Report Share Posted August 18, 2009 It is the stern post i am enquiring about not the bow I.m building the boat and I just thought it must be easier to fit the stern tube to a flat plate and wondered if turbulence would be a problem. I didnt personally think it would be but I knew that the many years of experience on here would come up with the pros and cons, thanks for your input It will have to be pointy then If you are doing the building yourself the easiest option might be to have a flat bit just big enough for the stern gear and to make a point all the rest of the way. More fabrication, but almost the best of both worlds. OTOH, most builders these days seem to fit a weld-in ring for the stern gear into a hole cut with a gas axe into the pointed stern post. Depends on your ability with a gas axe I guess, and experience will count here- either at cutting the right size hole, or in welding up any oversize bits. Regards N It is the stern post i am enquiring about not the bow I.m building the boat and I just thought it must be easier to fit the stern tube to a flat plate and wondered if turbulence would be a problem. I didnt personally think it would be but I knew that the many years of experience on here would come up with the pros and cons, thanks for your input It will have to be pointy then If you are doing the building yourself the easiest option might be to have a flat bit just big enough for the stern gear and to make a point all the rest of the way. More fabrication, but almost the best of both worlds. OTOH, most builders these days seem to fit a weld-in ring for the stern gear into a hole cut with a gas axe into the pointed stern post. Depends on your ability with a gas axe I guess, and experience will count here- either at cutting the right size hole, or in welding up any oversize bits. Regards N Link to comment Share on other sites More sharing options...
nb Innisfree Posted August 18, 2009 Report Share Posted August 18, 2009 With a bit of care and patience you can cut a nice neat hole in a pointed swim with a hole saw, start with a pilot hole and make sure it is lined up then follow with HS, takes a while but well worth it. Make sure you have correct size saw as once cut it can't be enlarged I haven't done this myself but seen it done Link to comment Share on other sites More sharing options...
Timleech Posted August 18, 2009 Report Share Posted August 18, 2009 With a bit of care and patience you can cut a nice neat hole in a pointed swim with a hole saw, start with a pilot hole and make sure it is lined up then follow with HS, takes a while but well worth it. Make sure you have correct size saw as once cut it can't be enlarged I haven't done this myself but seen it done Unless it's a very blunt swim, I would expect you to run out of depth with the (ordinary) hole saw long before you get to full diameter/width. Tim Link to comment Share on other sites More sharing options...
RLWP Posted August 18, 2009 Report Share Posted August 18, 2009 Unless it's a very blunt swim, I would expect you to run out of depth with the (ordinary) hole saw long before you get to full diameter/width. Tim Arbour extensions Richard Link to comment Share on other sites More sharing options...
Iain_S Posted August 18, 2009 Report Share Posted August 18, 2009 Arbour extensions Richard But once you've reached the depth of the actual hole saw, you can't go further. (Unless you can remove the core from the initial cut, which in this case would also remove any means of locating the saw) Iain Link to comment Share on other sites More sharing options...
RLWP Posted August 18, 2009 Report Share Posted August 18, 2009 But once you've reached the depth of the actual hole saw, you can't go further. (Unless you can remove the core from the initial cut, which in this case would also remove any means of locating the saw) Iain Damn. He's right you know... Richard Link to comment Share on other sites More sharing options...
nb Innisfree Posted August 18, 2009 Report Share Posted August 18, 2009 (edited) Damn. He's right you know... Richard Yes it was quite a blunt swim, saw him start it and later on saw the result which was very neat, didn't watch him throughout, but I recall him saying it was no problem. Must admit I can't remember if it was a longer saw or not Edited August 18, 2009 by nb Innisfree Link to comment Share on other sites More sharing options...
Timleech Posted August 18, 2009 Report Share Posted August 18, 2009 Yes it was quite a blunt swim, saw him start it and later on saw the result which was very neat, didn't watch him throughout, but I recall him saying it was no problem. Must admit I can't remember if it was a longer saw or not I've never seen 'deep' holesaws. You can get annular (Rotabroach) cutters which go a bit deeper than your normal holesaw but from memory even those only go to maybe 60mm deep - plus the sort of size needed for sterngear would cost more than the sterngear!! Tim Link to comment Share on other sites More sharing options...
Swallowman Posted August 18, 2009 Report Share Posted August 18, 2009 Damn. He's right you know... Richard No he's not, you just use the outside of the arbour to locate it into the bit you have already cut out. I have drilled through 2" timber using a 1/2" deep hole saw. you just take the cores out as you go. You just have to make sure you have a decent hole saw to start with; not one of those dangerous things with different diameter curved blades that fit into a single boss. Link to comment Share on other sites More sharing options...
Iain_S Posted August 18, 2009 Report Share Posted August 18, 2009 No he's not, you just use the outside of the arbour to locate it into the bit you have already cut out. I have drilled through 2" timber using a 1/2" deep hole saw. you just take the cores out as you go. You just have to make sure you have a decent hole saw to start with; not one of those dangerous things with different diameter curved blades that fit into a single boss. I don't think that would work from the outside, as there wouldn't be a full circle to locate the saw.Might be possible from the inside, using the pilot drill for location. Iain Link to comment Share on other sites More sharing options...
soldthehouse Posted August 18, 2009 Author Report Share Posted August 18, 2009 If you are doing the building yourself the easiest option might be to have a flat bit just big enough for the stern gear and to make a point all the rest of the way. More fabrication, but almost the best of both worlds. OTOH, most builders these days seem to fit a weld-in ring for the stern gear into a hole cut with a gas axe into the pointed stern post. Depends on your ability with a gas axe I guess, and experience will count here- either at cutting the right size hole, or in welding up any oversize bits. Regards N If you are doing the building yourself the easiest option might be to have a flat bit just big enough for the stern gear and to make a point all the rest of the way. More fabrication, but almost the best of both worlds. OTOH, most builders these days seem to fit a weld-in ring for the stern gear into a hole cut with a gas axe into the pointed stern post. Depends on your ability with a gas axe I guess, and experience will count here- either at cutting the right size hole, or in welding up any oversize bits. Regards N Are you saying in effect a flat stern post and then fixing a fairing to smooth the flow of water rearwards towards the prop, if so BRILLIANT if not Ive just thought of something briliant Link to comment Share on other sites More sharing options...
nb Innisfree Posted August 19, 2009 Report Share Posted August 19, 2009 With a bit of care and patience you can cut a nice neat hole in a pointed swim with a hole saw, start with a pilot hole and make sure it is lined up then follow with HS, takes a while but well worth it. Make sure you have correct size saw as once cut it can't be enlarged I haven't done this myself but seen it done Apparently the hole can be finished from the inside Link to comment Share on other sites More sharing options...
soldthehouse Posted August 19, 2009 Author Report Share Posted August 19, 2009 Apparently the hole can be finished from the inside First thoughts are is it done in stages and if so how, my powers of comprehension seem to be diminishing after years of solvent abuse at work or perhaps its the onset of the big `A` Link to comment Share on other sites More sharing options...
nb Innisfree Posted August 19, 2009 Report Share Posted August 19, 2009 First thoughts are is it done in stages and if so how, my powers of comprehension seem to be diminishing after years of solvent abuse at work or perhaps its the onset of the big `A` First, drill pilot hole from outside and then use hole saw to its max depth, go into engine room and use saw in the same hole to finish cutting. Done. You need to go very slowly and keep drill steady, maybe need a longer drill bit for second stage and perhaps a right angled drive if space is tight. Link to comment Share on other sites More sharing options...
soldthehouse Posted August 20, 2009 Author Report Share Posted August 20, 2009 First, drill pilot hole from outside and then use hole saw to its max depth, go into engine room and use saw in the same hole to finish cutting. Done. You need to go very slowly and keep drill steady, maybe need a longer drill bit for second stage and perhaps a right angled drive if space is tight. Thanks, there is so much going on at the moment I seem to have a wall up against incoming info, perhaps its a safety thing, thanks for that, regards, stuart Link to comment Share on other sites More sharing options...
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