Displacement Hull Power Catermaran

One thing that hasn't been mentioned, is that the OP stated at the start that this design is to operate slightly above Froude number. That is the worst operating speed target. That is the speed range where the economy is worst and the bow is pointing at the sky. Another issue is that the OP calls a vertical bow stem "wave piercing". Wave piercing bows are sophisticated designs that can't simply be attached to a boxy hull without rocker. This is the typical result of starting drawings without first creating an SOR.

 

I was asking what would happen IF I operated above the displacement hull speed. Design speed for efficiency was always at 7mph but was wondering what would happen if I went above. As a side note, when pontoon boats go past the froude speed, their bows do not raise that much. They raise a bit but not like a planing v-hull. Also from reviews the Glacier Bay displacement hull does not raise much either when they push past the froude speed. When I look at performance reports for these displacement hulls fuel economy pretty much shows this. There is no big improvement in fuel economy that you see once a planing boat plans off. Witch makes sense since they never plan off.

Regarding SOR, an important requirement is easy to build, that is one reason I chose that shape. Another was "slow and steady".

 

Here is an example of a displacement hull just getting steadily worse and worse as you push past the froude speed.

 

Yes, according to the internets, the Glacier Bay 2270 hull weighs 3600 lbs. An aluminum version made from .125 thick throughout would be 2300 lbs. Though copying the shape of a fiberglass hull is not always realisticly possible with aluminum. There are plenty of used Glacier Bays for sale once you get near Florida, but pretty much none, ever, in New York. I was down in Florida in February and could of looked at them at that time, but had not heard of that option at that time.

 

A properly trimmed planing hull will not raise the bow. Go online and check youtube videos of racing boats.

 

0.125" is 'only' 1/8", or 3 mm thk plating - what are your transverse frame spacings, and what thickness are they? And are you running longitudinal stiffeners as well?

The cat in Florida that I linked to previously is (perhaps surprisingly) still apparently for sale -
2001 Glacier Bay 2270 Isle Runner Power Catamaran for sale - YachtWorld https://www.yachtworld.com/yacht/2001-glacier-bay-2270-isle-runner-8245111/
It is a long way for you to fly down to have a look at it, but maybe somebody on here might live not too far away, and be happy to go and have an initial look at it for you? At the least, I think it would be worthwhile asking the broker for additional info. And she is already in Florida, so you dont have to trailer her down there.

 

I am aware, lived on a lake my whole life. I was simply responding to your comment stating my bow would be "pointing at the sky"....

 

2300 lbs is worse case, so I could make 3/16 work for the lower hull areas without gaining too much weight. But 3/16" is what the planing catamarans with twin 200hp outboards use so it seemed a little much. And the pontoon boat industry brags about using 0.09 and I know they dont have bulk heads or stringers between the sections. To answer your question, the current plan is to use bulk heads every 4 ft, with 1" flat stock stringers along the length of the boat. It would be nice if .16 was easier to get in the states. There seems to be more options in the metric world.

 

Yes, I have been eyeing it. But there are others for sale, so its not the end of the world if I miss this one. I need to see how much it would cost to store this boat in Flordia. My current tow vehicle is limited to 5000 lbs and its an awful time to buy a truck.

 

How did you calculate the weight of an aluminum

My comment is related to your target speed. It is right on the hump before planing. Faster or slower will be better.

 

Multihulls routinely sail past their displacement speed S/Ls, without climbing over their bow waves.

I don't imagine engine powered ones are that much different.

This may be due to the smaller bow waves they make, due to their hulls being very narrow.

I think it is safe to say that a catamaran, with two foot wide hulls, makes two bow waves that, even when combined, don't equal that of a four foot wide monohull.

Very narrow monohulls can behave the same way.

A Victorian Era steam turbine powered monohull that was 50 ft long reached over 17 kts. It was way too heavy and deep to plane.

Even with a 50 ft long WL, It was going well past an S/L of 2.0 in speed.

 

Have a look at the sharpie hull form, it is very similar to your design (but a real historical design):

Next make it as narrow/deep/long as you like, I used Carlson Design Hulls here (https://carlsondesign.com/hulls.zip).
Here is an a simple catamaran design:


And a outrigger pontoon:


Regards AlanX

 

Re Sharpii's observations above - the ally cat in my avatar is 49' long, and has very skinny hulls (their L/B ratio is approx 15 at the waterline).
She has displacement hulls, definitely not planing; with her original pair of 70 hp O/B motors she achieved a maximum speed of 16 knots with 10 people on board, and later on she achieved 25 knots when she was re-fitted with a pair of 150 hp O/B motors (again with 10 people on board).

 

Have you looked at the hull designs by Rodney Marsh , dart, tornado? These are very old sailing designs that similar hull are being used by super cat in South Africa as light disp power cat that use very little power
Supercat Marine https://supercat.co.za/
harder to build in aluminum but the full displacement design shows a 38’ cat do 20 knts with 2x50 hp outboards.
The plans for the original shape of the former Olympic class Tornado are availble .

 

One must wonder what happened to this plan.

I stayed away from it. But one thing bothered me. If you take a used 23’ planing cat and 8’ wide trailer version and put slow engines on it; that boat is only sea capable at say 110% of beam or 9’ seas. Without being able to get over the hump and go to port; it changes the operating paradigm by quite a bit. Even power at surf, etc. is a question. And then weather helm. If you power just enough to run at displacement; then encounter a 20 kt headwind; the boat would need some excess of power to not blow sideways. This gets into all sorts of issues not discussed here. The prop sizing for 15hp outboard would be limited by the plate and most likely props would be undersized for any weather except downwind.