Power Multi Hull Shape

[tropostudio]

In the low DL/L ratio power multihull area, it looks like there are a a couple approaches to hull section and profile. It's a small regime, so there aren't a lot of examples out refer to. My personal interests are in fuel-efficient cruising, moderate seakeeping, and transportability, which focuses on hulls in the 28-38' LOA range. I'd assume principles are transferable to proportionally larger hulls.

Power tri or proa hulls look to be distributed between 1) round-bilged hulls with accommodation flares similar to Iren's ILAN Voyager or Kurt Hughes's 38' Tri-trawler; 2) Shallow-V hulls such as TurnPoint Designs power tri or Paul Biekers's power proas; or 3) box-keel designs with pronounced strakes near the stern, such as the Bakewell-White Pacific Zulu power trimaran.

Selection of inboard vs. outboard propulsion would seem to be an arbiter. Hughes uses pod-mounted-outboards either side of the main hull, so a box keel wouldn't be advantageous. The Bakewell-White tri flares are abaft, providing width for a stern deck. Inboard power suits the box-keel, but I can't imagine the height of the stern strakes above the resting WL provides help with planing. In any design with strakes, you'd think that any energy put into wave-making that isn't put into decreasing displacement would be a net loss. Iren's boats look to be inboard powered, with hull flare going toward increased accommodation and perhaps to knock down spray.

Any thoughts as to the efficacy of one approach to another?

 

[Ad Hoc]

.... on hulls in the 28-38' LOA range...

With this constraint, 8.53m - 11.58m (in my language) this leads to the next one of:

........ in fuel-efficient cruising, ..

This means you need to aim for an LD raio of circa 7.5+

For the 8.53m hull (28') this gives a a displacement of 1.47 tonne
For the 11.58m hull (38') this gives a displacement of 3.68 tonne

So there is your starter for 10.

Design you boat to those 2 constraints, and your target displacement is given.

 

[tropostudio]

Thanks. I would anticipate a Fn range of ~0.7-1 for this type of hull. Regardless of WL length of cited examples, I'm asking in more general terms as to reasons for designs cited, which I'd surmise are operating at similar Fn's or DWL (Haven't tried to run numbers on that, so my bad if I'm way off). Wonder if it might be more as much about designer preference, aesthetics, and engine/prop installation as much as fluid dynamics.

 

[Ad Hoc]

With a Fn range of 0.7 -1.0 is ideal for a good displacement hull of the numbers I quoted above.

... Wonder if it might be more as much about designer preference, aesthetics, and engine/prop installation...

Correct.

As noted in my footer, A successful design - is greater than the sum of its individual parts....in other words, don't get bogged own with a single objective as the ONLY objective.
It isn't.. a good design is a blend of endless compromises to satisfy the SOR - objective.

 

[BlueBell]

a good design is a blend of endless compromises to satisfy the SOR - objective

"...endless..." ( Hence the design spiral )

And the more complimentary those compromises are to one another, the better!

 

[fallguy]

This is perhaps off topic, but a clear downside of outboards is they can drown.

And what do you mean by 'transportability'?

And is this only about tris?

 

[tropostudio]

fallguy - not so sure about the concern about outboards. Seem to power plenty of boats, inshore and offshore.

This started as a general inquiry into veracity of hull section choices for power multis that seemed to be operating under similar LDR/DLR and Power/Displacement ratios. I'll go back and try to find values for the cited examples, reverse engineering the numbers if required. But you all are right - SOR should drive all decisions, and now you have me thinking about 'The Boat.' Main use area would be Lake Superior - Apostle Islands. Into sailing, but as I age, I see more practicality in a fuel-efficient powerboat that could carry a couple of sea kayaks. Inland waterways or the Mississippi River are an option. So...

'transportability' means re-location via full-size PU truck and trailer. Not for frequent transport, but to get it from A to B without excessive gymnastics. Cat, Tri, Proa - there are great examples of each. I'd be designing and building this myself. Been awhile since I've done it, but I have the space and tools.

I'll toss out this power proa designed and built with high school kids for a solar-electric boat competition in 1996:

Electric Eel - 2 pics

• tropostudio
• Oct 28, 2020

PV + Group 27 lead-acid battery outrigger/proa configuration electric boat designed and built in...

Stress-formed 4mm and 3mm ply for the vaka and ama. 19.5' (5.94m) LWL at vaka. Ama was a 2/3 l & w scaled version of vaka. ~ 520 lbs lightship, 700 lbs (320 kg) at DWL. Vaka picked up ~525 lbs, ama ~ 175 lbs at DWL.

That gives an LDR of 8.7 for total displacement relative to Vaka, or ~9.2 for LDR Vaka alone or ama alone relative to apportioned displacement. It was Power limited to 24VDC x 60 A = 1440w. Boat made 6.5 knots at 1.93 Hp, at a Fn of 0.44. Slippery hull - sure it could have gone faster with more power. Belt reduction drive let us run an almost-square prop at around 1000 rpm. Faired skeg worked well. Rudder was deep, but kicked up. Bottom of prop was vulnerable, but bottom edge was about level with maximum hull depth.

If one was to build a cylinder-molded or stress-formed ply hull in the 32-38' (10-11.5m) LWL range, I think this would scale. My gut says we wrung about as much shape as we could out of the single skin ply in the little proa.

 

[fallguy]

The best choice is the Skoota 28 or 24.

I am building the Skoota 32 demountable about 10 miles from you. Crane costs are not cheap. The Skoota 28 is far more affordable and transportable. The demountable can come apart, but I only did so to move to Seattle and now headed to Texas.

the 24 is a folding boat and needs some adaptation for me to like it (2 engines a/o engines on the hulls).

 

[tropostudio]

Images below are of the Hughes 38' 'Trimaran Trawler'. I believe the rendering is updated from the as-built version, with longer amas mounted further aft. The hull cross section shows a shape similar to what can be achieved with tortured ply or cylinder molded ply. It scales almost identically to the max section we obtained on the proa. Numbers Hughes publishes for the boat:
37.5' LWL (11.4m)
5278 lbs (2.4 m^3) displacement
12.3 knots with 2 x 15HP outboard at full throttle

Giving:
LDR = 8.54
Fn = 0.60 at 12.3 knots
LWL/BWL (slenderness ratio) looks to be ~13.5/1 . FWIW, both hulls on the proa above have L/B of 13/1. Hughes has a lot of cylinder-molded designs, and the proportions often carry through to composite versions.



Fallguy - so you are building in Minnesota? If so, I'd like to see it! I have looked at the Skoota 28. The accommodations are great for the length and displacement, and I like the very reasonable approach to power vs performance. I must admit an affinity for a long, skinny tri or proa that can be made narrower without breaking the package into separate pieces for transport. Something just bugs me about all that hull volumes on the Skoota 28 being basically 'dead space', although I totally understand the reason for it.

 

[fallguy]

Images below are of the Hughes 38' 'Trimaran Trawler'. I believe the rendering is updated from the as-built version, with longer amas mounted further aft. The hull cross section shows a shape similar to what can be achieved with tortured ply or cylinder molded ply. It scales almost identically to the max section we obtained on the proa. Numbers Hughes publishes for the boat:
37.5' LWL (11.4m)
5278 lbs (2.4 m^3) displacement
12.3 knots with 2 x 15HP outboard at full throttle

Giving:
LDR = 8.54
Fn = 0.60 at 12.3 knots
LWL/BWL (slenderness ratio) looks to be ~13.5/1 . FWIW, both hulls on the proa above have L/B of 13/1. Hughes has a lot of cylinder-molded designs, and the proportions often carry through to composite versions.

View attachment 169123 View attachment 169124 View attachment 169125

Fallguy - so you are building in Minnesota? If so, I'd like to see it! I have looked at the Skoota 28. The accommodations are great for the length and displacement, and I like the very reasonable approach to power vs performance. I must admit an affinity for a long, skinny tri or proa that can be made narrower without breaking the package into separate pieces for transport. Something just bugs me about all that hull volumes on the Skoota 28 being basically 'dead space', although I totally understand the reason for it.

Richard designed a compromise in the Aegean. 30' iirc

You can private message me your contact info and we can setup a visit.

 

[tropostudio]

Another example in the same size range as the Hughes 38' Tri-Trawler is the Bakewell-White Pacific Zulu power tri:

Limited info I can find shows:
14 knots max, 10 knots cruise, from a 54 Hp Yanmar diesel inboard.
3600 kg (8000 lbs) displacement.
12m (39.5') LOA and 0.5m (1.7') draft

From this we suppose:
LDR = 7.8
Fn = .66 at 14 knots
Fn = .47 @ 10 knots

I can't find images to determine shape of underwater section (round bilge, flat-panel, whatever...). I assume the canoe body below the flared strake is about increasing accommodation in the main hull that transitions to a functional aft deck, than it is about providing lift at speed. It looks way too high above the DWL to act as a a lifting surface. A couple internal pics show the canoe body narrowing at the transom. Perhaps to keep prop shaft level with the inboard engine as low as possible?

 

[tropostudio]

Ad Hoc -

I appreciate your initial advice as to looking for LDR 7.5-1. That applies to boats squarely in the 'Semi-Displacement' range from the literature I have available. Understood that whether a hull is displacement or planing is open to debate. I don't care whether a boat is planing or not. I understand wringing every last bit of efficiency for really low power is hard work, and that a making a boat plane means relatively high power/weight ratio.

Somewhere I recall Tennant using the the immersed canoe body with planing strakes at DWL approach for power multis. I was wondering if this was the intent with the Bakewell-White 'Pacific Zulu", but figured not, considering the low power/displacement ration and how high the strake sits above DWL. Got me to thinking about a couple of Phil Bolger power designs - Hawkeye and MicroTrawler - that used box keels and 'planing strakes' right on the DWL to get shorter hulls up on plane with lower horsepeower. One would think the added wetted surface of the strakes just adds wetted tresitance until the hull is moving fast enough and trimmed properly for them to act as lifting surfaces.

Do you see much advantage to the round-bilged approach of something like the Hughes Tri-Trawler vs the Bakewell-Wight vaka form, other than ability to accommodate outboard vs. inboard power? How does wave action on the underside of the wide stern platform on the Bakewell-White hull contribute to hull drag, trim, comfort/seakeeeping?

 

[Mr Efficiency]

The Tennant canoe stern had a wide flat area around the DWL aft, probably to damp pitching motions, and might have acted as an anti-cavitation aid, at a stretch

 

[tropostudio]

Ahh, that makes sense. The Bakewell-White tri's stern deck looks pretty high above the DWL. It would probably mitigate stern squat, depending on height of bow wave when it gets back there. And pitch damping, for sure. I'd like to video of the boat at speed, and pics that would give a better sense of hull shape. The pic below of the engine installation show what looks to be a very narrow immersed canoe body that pulls in toward the stern.



The Hughes Tri-Trawler hull looks to be fuller, but has a shape more typical of a sailing multi, with transom just clearing the water. I think I'd prefer the lower stern platform of Pacific Zulu for easy water access, and its raised helm position than standing on top of the Hughes deck. His drawings show a seated helm poistion inside the cabin, just forward of midships.
,

 

[Mr Efficiency]

The Tennant stern is adapted for inboard propulsion, and gives nice prop protection and reduced draft, don't know how outboards would go, probably not as well.