Shanty boat build - self design BBC materials

[jbo_c]

The gt23 is a great design with a larger displacement houseboat hull option.

I had an old Evinrude 18 and it died when I forgot to add oil to a 6 gallon tank fill up. Broke the crank right in half.

Interestingly, both the GT hulls have immersed transoms/are planing hulls unless I’m very mistaken. I’m actually considering considering a GT27 build when I retire depending on physical ability and monetary comfort level.

Love my little 18. It’s been going as long as I have. I’d be heartbroken to lose it.

Jbo

[Jaysen]

I don’t think transom immersion defines planing or displacement. If it did sailboat hulls designers would have a lot of explaining for how many are incorrectly designed.

The shortest answer is that the hull shape, things like planned rocker vs flat to transom, is what differentiates planing vs displacement. Jacques has often commented that planing hulls can be displacement hulls much much easier than displacement hulls can be used as in planing hulls. His advice in the past has been to ask his direct advice on how to use a planer in displacement context.

I’m not sure if Jacques would approve of the XF as a displacement platform but it seems to me that it would be easier to build and hence worth asking Jacques about.

[jbo_c]

I don’t think transom immersion defines planing or displacement. If it did sailboat hulls designers would have a lot of explaining for how many are incorrectly designed.

The shortest answer is that the hull shape, things like planned rocker vs flat to transom, is what differentiates planing vs displacement.

I understand the difference. But folks here keep recommending I use a hull designed for planing for a program plan for displacement, which just doesn’t make sense to me, so I’m trying to understand why.

Jbo

[cracked_ribs]

I wouldn't say transom immersion is necessarily what defines a planing boat per se but an immersed transom generates a lot of drag and I can't think of anything designed to run at displacement speeds with an immersed transom other than a barge.

If you look at the same boat in planing and displacement forms...say Ross Lillistone's Fleet and Flint - the big difference is that one has enough rocker at the stern to pull the transom out of the water. If you leave it in, it's going to suck, literally.

An immersed transom needs enough speed for the water to separate cleanly. That's not normally part of displacement operation, and definitely not on any typical hull form that I can think of. Maybe there's some freak boats out there that are exceptions, I'm not sure. But generally speaking, if you leave the transom in the water, it means that until the boat begins to plane you're dealing with a lot of suction on the transom.

You sometimes hear guys saying something like "the transom dries out at 11 knots" - this is essentially a description of planing.

Of course you can chug around in a planing boat - I sometimes do in my deep V pig. The xf would be way more efficient than that.

But a true displacement form, even more so.

[cracked_ribs]

Anyway in regards to this particular boat, I know that the tendency here is always going to be to steer people to one of the existing plans. I do think that the plans here are very good and owning a set or two is worthwhile from a study perspective.

But if you want something not quite identical to what's available here, for sure, design something yourself. There's always Dave Gerr's scantlings (which are total overkill on small boats), experience and imitation to guide you. People get really wrapped around the axle that you need every number worked out to five decimal places and every detail accounted for.

I would totally agree on any boat that will cross oceans or hit 50 knots or something.

A shanty boat in protected waters? Keep the weight down low. Look at proven scantlings from someone like Gerr. Enjoy the process. I find it really rewarding.

[jbo_c]

I wouldn't say transom immersion is necessarily what defines a planing boat per se but an immersed transom generates a lot of drag and I can't think of anything designed to run at displacement speeds with an immersed transom other than a barge.

If you look at the same boat in planing and displacement forms...say Ross Lillistone's Fleet and Flint - the big difference is that one has enough rocker at the stern to pull the transom out of the water. If you leave it in, it's going to suck, literally.

An immersed transom needs enough speed for the water to separate cleanly. That's not normally part of displacement operation, and definitely not on any typical hull form that I can think of. Maybe there's some freak boats out there that are exceptions, I'm not sure. But generally speaking, if you leave the transom in the water, it means that until the boat begins to plane you're dealing with a lot of suction on the transom.

You sometimes hear guys saying something like "the transom dries out at 11 knots" - this is essentially a description of planing.

Of course you can chug around in a planing boat - I sometimes do in my deep V pig. The xf would be way more efficient than that.

But a true displacement form, even more so.

Thanks, Cracked.

Those are the points I was trying(perhaps unsuccessfully) to make.

Jbo

[cracked_ribs]

Well nobody is obligated to agree with me, but I think you have a good point! You could totally motor the XF around on a small engine - I have no problem pushing my two ton deep V with a 9.9 - but it's not ideal and if you want to design something yourself anyway, why not?

[TomW1]

Well said CR it is his boat to build. I was trying to get him a proven hull to build that would get him off to a great start and hold his shanty. jbo just make sure your confident in your scantlings, weight and flotation calculations. Your PPI should be closer to the XF22 at 627lbs than the HDM19 displacement at 320lbs my last advice to you.

Good building and good luck.

.Tom

[cracked_ribs]

Yeah that's a fair point about the XF22 - at least it's got all the scantlings totally ready to go and you know the parts will fit. It's not a bad choice at all.

[jbo_c]

I do really appreciate everybody’s help. Can’t wait to start the build.

Already 5 pages into my thread and don’t have the wood yet.

Jbo