SWIM DESIGN

[AllanC]

There was a fair bit of emphasis on good swim design being important in the 'Power in Reverse' topic. So what makes a swim a good or bad design. If I were designing my own 60' cruiser stern, how would I design the swim for the best performance.

[NickF]

I am by no means an expert and no doubt others will come along with more expert knowledge. However I always thought the longer the swim the better the boat went through the water, the problem being it will have some impact on cabin space if it is too long. So it's a compromise between space and good boat handling and so must be affected by what you want to use the boat for, if it is not to be moved much have a short swim and more space inside.

[blackrose]

In general longer swims are seen as better than shorter ones but I'm not sure what the ratio should be?

[blackrose]

This alternative stern design isn't as common as the standard vertical swims we see on most canal boats. The advantages of the design are that swims can be made shorter and perhaps it allows more water to get to the prop, but it's probably more time consuming and expensive to build and engine positioning may need to be further forward, so perhaps you lose the benefit of shorter swims?

Edited August 3, 2017 by blackrose

[mrsmelly]

25 minutes ago, AllanC said:

There was a fair bit of emphasis on good swim design being important in the 'Power in Reverse' topic. So what makes a swim a good or bad design. If I were designing my own 60' cruiser stern, how would I design the swim for the best performance.

Longer swims means for instance a slender profile beneath the water hence more grip for the prop at the arse end as more water round it especialy for stopping power. My Hudson had very long swims fore and aft but this meant swims went several feet into the rear bedroom and front saloon, they were approx 13 feet long. I lost some width to the bottom of the rear double bed and in the front saloon behind the stove even though the front end outside was 11 feet long. Everything in boats is a compromise.

[Nightwatch]

Shallow water and prop drift makes a difference. Regardless of swim. In my view.

[DHutch]

There's a million things that effect how a boat swims through the water, but in terms of swim design adding length is certainly one of the easier options in terms of fabrication.

The other thing that's nice is to pull the bottom in a bit harder than the top edge to get the same sort of shape as on the historic boats, FMC/GUCCC etc. Harder to do as a one off in sheet steel than riveted plate with templates however. Replacing the hard edge with a 4inch radius would also be nice but I've never seen it.

 

Daniel

[zenataomm]

Make your swim 30' so it bellys out in time to meet your 30' double curvature front end bending the other way.

You then buy all your fit-out furniture from Fyffe, and use boomerangs as shelves. 

[IanM]

Swim on a historic boat

Nutfield - Braunston Dry Dock 16/10/2016 by Ian, on Flickr

Swims on a lot of modern leisure boats tend to have vertical sides.

[alan_fincher]

58 minutes ago, DHutch said:

The other thing that's nice is to pull the bottom in a bit harder than the top edge to get the same sort of shape as on the historic boats, FMC/GUCCC etc. Harder to do as a one off in sheet steel than riveted plate with templates however. Replacing the hard edge with a 4inch radius would also be nice but I've never seen it

 

 

 

 

[Sea Dog]

2 hours ago, AllanC said:

 If I were designing my own 60' cruiser stern, how would I design the swim for the best performance.

It's an interesting question but, if you need to ask it, best let someone else design your boat!

[NickF]

On 03/08/2017 at 14:25, alan_fincher said:

 

 

 

That's really beautiful. (Is it quite sad to think that?)

[Boat&Bikes]

7 minutes ago, NickF said:

That's really beautiful. (Is it quite sad to think that?)

Although I have no particular interest in older boats I find this hull shape beautiful as well. Looks as though it was built to go Fast!

Ian.

[zenataomm]

17 minutes ago, ianali said:

Although I have no particular interest in older boats I find this hull shape beautiful as well. Looks as though it was built to go Fast!

Ian.

More a case of built to go efficiently, something few boats built these days can boast.

[AllanC]

2 hours ago, Sea Dog said:

It's an interesting question but, if you need to ask it, best let someone else design your boat!

I agree. Its doubtful I would ever have the expertise to design a boat myself. It would be more a case of presenting the idea to a boat builder and seeing what they could come up with. I would not be aversed to having a longer rear deck if that was required, even at the expense of a couple of feet of cabin space.

[Boat&Bikes]

18 minutes ago, zenataomm said:

More a case of built to go efficiently, something few boats built these days can boast.

Yes agreed, but it looks Fast!

Ian.

[Neil2]

Interestingly, on another thread recently I was making the assertion that there are very few genuine boat builders in the narrowboat industry now, it's mostly populated by skilled fabricators.  Someone replied with the question what's the difference between a fabricator and a boat builder, I think the pictures above say it all.

 

[X Alan W]

The shape of historic boats under water can be reproduced but it's time consuming, = extra cost & only a few shell builders seem prepared to take it on  the modern way appears to be attained with shaped strips of plate rather than just pulling the sides in to meet/attach the stem post & either pushing up the bottom or fitting 2 triangular strips, as modern hulls seem to be Longer swims & shaped as per historic boats tend to perform better but interior space seems more desirable than efficient swimming, & the state of the cut re depth possibly cancels the finer shape, I know that on deeper water ie rivers Soar, Trent, etc. my FMC motor would outrun boats that had been keeping up on the cut.

[David Schweizer]

The swims on Helvetia were 15ft fore and aft on a 52ft hull, so an aproximate ratio of 2:1 at each end. With a draft of 33" underway, she swam beautifully, and steering backwards was very easy. The  downside was that with such a big rudder, steering was heavier than on many modern shallow boats, and fitting out at both ends was a bit of a challenge, because the swims intruded into the front and back cabins. but it was worth it for the handling properties.

Edited August 3, 2017 by David Schweizer

[GUMPY]

The swims on Loddon are fine enough for me, not the usual slab sided modern boat 

Swims like a fish with almost no wash and will reach 7mph with ease.

[IanD]

It's not just the swim that affects steering in reverse, it's also prop diameter and position -- a small prop too close to the hull deadwood and too far from a small rudder won't steer well especially in reverse, unfortunately this is what many modern boats have as well as short swims. What you want is a big prop close to a big rudder (a couple of inches clearance) and well clear of the hull (5"-6"), this makes the rudder guide the water flow into the prop in reverse and gives enough room in front of the prop so that more water can go to one side of the hull than the other when the rudder is to one side. It's what all the books on ship hull design say should be done, and very few narrowboats do...

[AllanC]

  There is no denying that Loddon has nice curves at the bow. Makes that boat behind look pretty boring. I see what you mean about the sloping sides on the swim too.  Maybe I just need to find a well built old boat instead of a new one.

[Neil2]

Our boat has a long "raked" swim, not quite as pronounced as the boats pictured above, and I bought it on the strength of pictures I saw of it out of the water - how many boats for sale do you see with a picture of the hull?   But, this is the point, it's a middle market boat from the 1980's.   If you buy a middle market boat today I suspect you will get 90 degree angles and the shortest swim the builder can get away with so the fitter out can cram as much as possible inside.  

I was heavily influenced by a lengthy post on here many years ago just after I joined the forum.  Until then, like most people who were attracted to narrowboats I didn't really appreciate the difference in quality between boats, or how to identify it.  The originator of the thread asserted that the cost of the shell, and in many cases the reputation of the builder, didn't really count for very much and if you were going to spend a lot of money on a "quality" boat you ought to learn as much as you can about the basics because there's a lot of smoke and mirrors in today's industry and a lot of camouflage.  

Some people end up with a good cheap boat by accident, but there are ways of doing it by design if you take the trouble to learn.  I have to say I'm still learning...

If you want a classy underwater shape these days there are maybe a handful of builders who can do it for you but you need deep pockets.  The good news is most narrowboat buyers are pretty clueless, or just don't care, as to what's going on down below so occasionally a really nice boat comes on the market and doesn't command a heavy premium even if it is well built because most folk don't look past the fit out.    

[blackrose]

23 hours ago, Neil2 said:

Our boat has a long "raked" swim, not quite as pronounced as the boats pictured above, and I bought it on the strength of pictures I saw of it out of the water - how many boats for sale do you see with a picture of the hull?  

Are you talking about a new build or a second hand boat? Perhaps I've misunderstood but either way I'd want to see more than pictures before committing to buy. If it was used I'd want to see the boat out of the water during a survey. Some have got away with it, but we've all read about "clueless" narrowboat buyers on this forum who've neglected to have a survey on a used boat and later regretted it.   

Edited August 5, 2017 by blackrose

[Neil2]

6 minutes ago, blackrose said:

Are you talking about a new build or a second hand boat? Either way I'd want to see more than pictures before committing to buy. If it was used I'd want to see the boat out of the water during a survey.

I'm talking about used boats here, the point I'm making is that most people make a purchase decision based on what they see above the waterline.  I was lucky in that the owner of our present boat had taken photographs the last time it was out of the water, so you could see the shape of the hull, swim, position of the prop etc.  That was a major factor in deciding to buy it.   The vast majority of buyers will put down a deposit subject to survey then the boat will come out of the water but it's primarily to check on the integrity of the hull not the design.  Most buyers I suspect don't even bother to go and look at the boat on the hard, just leave it to the surveyor.  

It's different in the lumpy water boat world, where, if you fancy a particular boat there will probably be a picture of it out of the water, or, you already know what the hull looks like because there are hundreds of similar boats around.   Narrowboats are completely different yet I never heard of anyone wanting to see one out of the water before making a purchase decision.