Most Efficent Low-Speed Hull

[handmer]

My question is, what is the most efficient hull shape for low speed use. Around 5 knots.

I am aiming for a sub 6m length. As little draught as possible.
For use on very protected waters only. No wave handling is really needed.
Total weight around 1000KG. Ideally a multi-hull for wide tramp decks.

The idea is to use electric propulsion and a crab claw sail for downwind work. Plus a backup outboard shared with the dinghy!


I have done a lot of reading into hull efficiencies with different shapes but not a lot seems to deal with low speeds.
So, does anyone know what hull shape is most efficient at low speeds? I know any skinny long hull is pretty good, but there seems to be a lot of opinion on entry and exit shapes etc.

[Tropic Cat]

You might consider posting your question over there:

Boat Design Forum

As they do specialize in boat design

[handmer]

Thanks, that's a good idea!

[Maren]

Originally Posted by handmer

My question is, what is the most efficient hull shape for low speed use. Around 5 knots.

I am aiming for a sub 6m length. As little draught as possible.
For use on very protected waters only. No wave handling is really needed.
Total weight around 1000KG. Ideally a multi-hull for wide tramp decks.

The idea is to use electric propulsion and a crab claw sail for downwind work. Plus a backup outboard shared with the dinghy!

I think you'll find a pontoon boat is about what you are looking for given the mix of weight, draught, speed. Here are two, a run of the mill pontoon boat and, in the thumbnail, a Gougemaran (source website but you can find two in the 5th edition of the Gougeon Bros on Boat Building). Read the caption under the latter if you will.

[dustman]

Originally Posted by handmer

My question is, what is the most efficient hull shape for low speed use. Around 5 knots.

I am aiming for a sub 6m length. As little draught as possible.
For use on very protected waters only. No wave handling is really needed.
Total weight around 1000KG. Ideally a multi-hull for wide tramp decks.

The idea is to use electric propulsion and a crab claw sail for downwind work. Plus a backup outboard shared with the dinghy!


I have done a lot of reading into hull efficiencies with different shapes but not a lot seems to deal with low speeds.
So, does anyone know what hull shape is most efficient at low speeds? I know any skinny long hull is pretty good, but there seems to be a lot of opinion on entry and exit shapes etc.

Minimum wetted surface area and low prismatic coefficient.



A semi-circular underwater hull section has the least wetted surface area per volume. Wetted surface area is the main source of drag at slow speeds for any reasonable hull shape.



The low cp would basically mean smooth curves for the water to follow. Contrary to what would seem logical, water at slow speed has a harder time following a contour, than at higher speeds. That means more turbulence which increases friction.


I think a good question to ask may be what is a good displacement to length ratio for the speed in question.

[coopec43]

handmer asked that question in 2010. Maybe he is not like me and has already solved the problem!

Also he hasn't been seen on the forum since 2020

[dustman]

Originally Posted by coopec43

handmer asked that question in 2010. Maybe he is not like me and has already solved the problem!

Also he hasn't been seen on the forum since 2020

Replies can be useful, even with the OP long gone. What problem are you working on?

[coopec43]

Originally Posted by dustman

Replies can be useful, even with the OP long gone. What problem are you working on?

Still putting finishing touches (sail bags and LPG plumbing) to a project I started yonks ago.

[s/v Jedi]

Originally Posted by coopec43

Still putting finishing touches (sail bags and LPG plumbing) to a project I started yonks ago.

LPG plumbing!? …

[boatman61]

Originally Posted by handmer

My question is, what is the most efficient hull shape for low speed use. Around 5 knots.

I am aiming for a sub 6m length. As little draught as possible.
For use on very protected waters only. No wave handling is really needed.
Total weight around 1000KG. Ideally a multi-hull for wide tramp decks.

The idea is to use electric propulsion and a crab claw sail for downwind work. Plus a backup outboard shared with the dinghy!


I have done a lot of reading into hull efficiencies with different shapes but not a lot seems to deal with low speeds.
So, does anyone know what hull shape is most efficient at low speeds? I know any skinny long hull is pretty good, but there seems to be a lot of opinion on entry and exit shapes etc.

Red and white Hitia 17 on the beach
Building Method: Ply/Glass/Epoxy Stitch & Glue
Length Overall: 17' 5.18 m
Beam Overall: 10' 1" 3.07 m
Draft: 11" 0.30 m
Weight: 295 lbs 134 kg
Loading capacity: 550 lbs 250 kg
Sail area: 160 sqft 14.9 sqm
Building Time Estimate: 250 hrs
Hitia 17 is the perfect beach catamaran, but also capable for use as a micro Coastal Trekker. Her wingsail sprit rig with low centre of effort gives good drive and great stability. The sail can be reefed and hence the boat can always be sailed safely without the risk of capsize, which most other beach cats are prone to. She has skegs and rudders that do not project below the V-eed hull, hence she can be sailed straight off the beach without lifting rudders or daggerboards.

She has stowage space within the hulls and the two self draining 'cockpits' are big enough to sit in, or stow camping gear for a longer Trek. The boat can carry four adults. The deck is designed to take a standard two-man dome or pop-up tent, or you can make your own decktent to drawing supplied. Can be carried on the car roof of a bigger car or is easily trailed. A lovely minimalist coastal explorer.

The sail plan can be altered for Crab Claw rig if wished just contact Wharram for advise.

[Jim Cate]

You might look at rowing shell type hulls, for they have had decades of thoughtful development in the hands of international competitors.

Jim

[coopec43]

Originally Posted by s/v Jedi

LPG plumbing!? …

Low Pressure Gas, NO! We learn something new everyday

Actually Liquified Petroleum Gas "A mixture of flammable hydrocarbon gases"

[s/v Jedi]

Originally Posted by coopec43

Low Pressure Gas, NO! We learn something new everyday

Actually Liquified Petroleum Gas "A mixture of flammable hydrocarbon gases"

That’s from dinosaur squeezings, soon they will not allow you to have any like already in Europe…

[ggray]

Originally Posted by Jim Cate

You might look at rowing shell type hulls, for they have had decades of thoughtful development in the hands of international competitors.

Jim

This:

In general, the greater wetted surface of catamarans put them at a disadvantage in lighter airs.