Power Proa design

I am contemplating either a 30’ Power Proa or a Trimaran and want to know thoughts on efficiency and speed?
Here is the type of power Proa
20 foot Power Proa - Bieker Boats https://biekerboats.com/project/20-foot-power-proa/

 

So, are you thinking about extending that 20' Bieker boat to 30', and then possibly adding an extra outrigger to make it a trimaran?

Re thoughts on efficiency and speed, the trimaran would be a bit more stable re capsizing (but do you really need this?) and it will be heavier, with more resistance, and hence a bit slower.

 

This may be a philosophical question--more likely just one of definition--but can you still call a boat a proa if it has no sails, has a single-ended hull, and doesn't shunt?

 

Not sure, but that is what Russel Brown, Rob Denney and Paul Bieker call it…

 

Single outrigger. Thanks on Trimaran efficiency info!

 

Help me understand why a 30 foot power boat needs an outrigger.... unless the main hull is very narrow.

A 30 foot conventional hull that is five feet wide would, or at least could, be sufficiently stable. It would have a six to one length/ beam ratio and be easily propelled. A 30 x 6 layout could be even more stable and have a 5:1 ratio. Either boat could be lighter than the proa, more conveniently brought to a dock or pier, have more interior space, and so on.

 

Thanks mate, interesting that I received correspondence from Rob Denney, he stated the power proas are not stable- you have to have heavy arma and as a result produces more drag.
As a result, I am considering power trimaran.
Now, need to know how far out the amas need to be.

 

Your concept of multihull power boat is interesting. You must surely have a reason, or several reasons, for favoring a multi.

Cats and Tris, are most often used as sailboats. There is good reason for using outriggers for a sailing boat, not nearly as much reason for using outriggers on a power boat. The deal is that you can develop a lot of initial, or near initial, righting moment with the multi. The sail pushes sideways on the mast. The mast amounts to a big lever. That leverage tends to heel the boat. Sticking the ama way out from the center of the boat amounts to a restoring lever. All that is obvious of course. Multi hulled boats do not need ballast and can therefore be lighter. In general they will go faster than a ballasted monohull of the same length. Not so for powerboats.

A power boat does not suffer from the overturning moment generated by the sail and mast. The powerboat has little need for excessive righting moment that might be provided by outriggers.

But you may have a unique need. Please post a statement of requirements (SOR) for your boat. That involves anticipating the kind of water, or sea state that you anticipate, how long will your trips be, how many passengers will you normally carry, will you have toilet facilities, will you need refrigeration or large ice chests, what cruising speed do you wish to have as a norm, is top speed one of your priorities, will you have overnight accommodation such as a closed cabin, is economy of operation a priority, will you anticipate trailering the boat or will it have permanent mooring,and any other factors that are important for your boat.

That may seem a lot to ask but just imagine that you have engaged a certified naval architect to design and manage the construction of your new boat. He/she will absolutely require you to furnish a carefully considered SOR. Otherwise how could the NA guess what it is that you want and how to design the boat to fit your needs.

That is a long winded and circuitous reply to your question: "how far out should the amas be?" The answer to that depends to some extent on your SOR.

 

Thanks mate- I live on the Southern Moreton Bay Islands and want the boat for commuting to mainland to get groceries and bring petrol back in jerry cans. As the car ferry is $135.00 round trip, and you cannot bring petrol on the passenger ferry. Also, there is a guy who built a 21 foot boat with 4hp outboard that does the same thing to go to mainland. And limiting to 4hp avoids having to registering and paying fees.
So, I found a 7.5m 24 foot carbon fiber catamaran hull to use as my main hull for lightweight and narrow 2 foot wide in the middle and 2 foot high.
So, now my plan is to build a lightweight trimaran built around 4hp. Also, to beat his speed of 12-14 knots.
2 people total 150 kg
Carry groceries mostly
Bring back petrol every 2 months
Able to beach the boat to explore surrounding waters
Have a small cabin for sleeping 2
Finally, have fun on the water.

construction foam core 20mm sandwiched fibreglass. Aluminium outriggers.

 

A few thoughts -

First, your description and sharing of the intended use and reasons for building a Tri based on the existing hull you have is helpful.

The speed of the other boat using only 4HP seems kind of high, but maybe it’s not a Multihull and is a planing design?

Or, is it really a 5 or 6 in disguise?

If you don’t know, most 4-6 4 stroke motors are more or less the same with different size carbs/jets/props meaning you can swap a carb and or a prop and tap into the extra power. It’s also a way to save some money, then keep the original carb and or prop as a spare.

Anyway, one thing you might consider since this is a motor versus sail boat is to make your beams telescoping so you can reduce the width of the 3 hulls for “speed runs” and trailering if need be.

Then when you need extra stability or more room for passengers/payload/lounging, you can extend the beams.

Take a look at Revintage’s build under the Multihull thread.

He is building a sailing Tri that may provide some inspiration.

Also suggest searching “FrankenTri” for examples of Trimarans made using repurposed Catamaran hulls.

 

In your previous thread you seemed very keen on a slightly larger version of @HJS Keyhaven skiff - have you changed your mind, or are you just looking for other possibilities as well now?
VSV LDL design https://www.boatdesign.net/threads/vsv-ldl-design.67987/

Re your desire to have a maximum of 4 hp - is the registration process for boats in Oz very onerous and costly?
If you had say a boat with 20 hp instead of 4 hp, what would be the difference in the amount of fees that you would have to pay annually?

 

Thanks Forecaddie. Your aim to build a tri now makes sense. Since you have the main hull there is perfectly good reason to make it a viable boat. Sure enough a hull as narrow as that narrow is going to need some help for staying right side up. Outriggers can make it so.

Going back to your question....."how far out do the outriggers need to be". Think of the boat as if it was a playground see- saw. The middle hull is the pivot point of the see- saw. (I don't know what Aussies call that playground pivoting board, perhaps teeter totter or something else) The see saw is a case of leverages. If a 80 kg person is on one end and a 40 kg person is on the other end, then the pivot point needs to be moved in order to achieve a balanced system. The lighter weight person needs to have more leverage if we are to achieve balance. What you have to do for the boat is calculate the distance from the center of the main hull to the center of the outrigger floating body. If you use a small ama then the distance from center to center needs to be large. If the ama is large then the distance can be less.

Alas........ it is not so simple to calculate the needed righting moment of a tri as it is to calculate the see saw problem. The ama can have different buoyancy depending on how deeply it is immersed.... thus how much it pushes upward on the crossbeams so as to resist the roll attitude of the main hull. A lot of this plays into the condition or roughness of the water where you are traveling and also on your tolerance limit of roll. There is more......If you are heading into or nearly into a head sea then the amas are not having to work very much. If you are running parallel to the wave train then the boat is going to rock and roll regardless of the extra righting moment caused by the amas even though the amas are doing their job. SolGatos suggestion may have some merit. determine the leverage of the amas by having adjustable length beams. That would invovolve some experimentation and possibly some thrills if you got into a hard chance with narrow beam length.

It is worth mentioning that the amas are usually arranged in such a way that one of them is above the water while the opposite one is in the water doing what it does. You are going to use only modest propulsion, like 4 HP. It is then essential to reduce the wetted surface of the boat as much as is feasible. (Wetted surface: the parts of the boat that is underwater and the water is always "rubbing" on the submerged surfaces of your boat) Lots of wetted area saps horsepower in order to overcome what you can think of as friction. SO when you build the amas that needs to be kept in mind. Heavens to Betsy! Why is boat design so fraught with variables???

You still do not have an answer to your perfectly legitimate question. We would have to know a lot more about the main hull and a lot more about how much heel you can tolerate in a particular set of sea conditions.

 

Such great advice mate! And will look into your suggestions. His boat is 3’ wide 21’ foam sandwich with fibreglass. My hull is very narrow and longer so stability is an issue! Again great forum here!

 

Check out bangka racing in the phillipines on YouTube