2.5hp Catamaran - power delivery issue

Hi, I'm new to boatbuilding and this site. I recently had the idea of trying to make a boat that planes with a 2.5hp engine. It appears that even with only one person on board this is quite a challenge.
So the design brief is to make something car roof topable, weighting less that 30kg, costing less than £300, and importantly going at least 10mph (hoping for 12-13mph).

I decided to purchase an older style longboard windsurfer. I sawed it in half and built a temporary wooden deck to separate the two hulls by about 60cm. It is 3.3m long and has enough buoyancy. It is stable and weights less that 30kgs. The problem that I have is that is performs well up to a point but as the revs increase the engine just produces a 'confused' wash and there is very limited forward propulsion. I got it up to 8.8mph but it wasn't pretty. At first I clearly had the engine too high and was experiencing cavitation so I lowered the engine by 4" (as seen in the photo) which I thought would solve the problem. It did seem to stop the cavitation issue but not the confused wash.

It is difficult to see exactly what is happing because I am unable to position myself far enough back without making things worse. Incidentally I sit sideways on it with feet resting on portside hull. Ideas I've had are as follows but I'm really at a loss;

1) engine still too high (but transom height is less than 30cm from water and anti-cavitation plate is below hulls)
2) engine trim not correct (it is only tilting forward 10-15 degrees)
3) inside edge of each hull (which still has the recess for the centreboard) generating erratic wash in front of the engine.
4) Hulls too close together and as above disturbing water flow
5) Transom raised above waterline (with no hull bottom to flatten the water) so too much of the engine is cutting through the water and creating turbulence.

Any ideas would be greatly appreciated. I really want to take this design to the next level.

P.S. I should mention that I did try moving the whole deck back about 10" to see if moving the engine back would help, but it didn't make any difference. NB the photo is of the engine in the original position.

 

The hulls may be generating lift resulting in the immersion of the outboard leg being inadequate when coming up on plane. One cant tell by looking at the static floatation level... You could make yourself a hydrofoil to bolt onto the outboard leg. They dont seem to be available down to 2.5hp in the US, cant tell for the UK... The Australians seem to have them, quite pricey at $110 Aus. All you need is a bit of 1/8" maring grade aluminum plate of the right shape and some fasteners...

But try just dropping the outboard lower first.

 

The old, out of production XS speed sailboard has a similar shape with it's straight tunnel and curved outside hulls. The first image shows the flow pattern behind and it appears to be planing. Maybe you just need a bigger motor, to get past the turbulence hump?

XS speed sailboard at DuckDuckGo https://duckduckgo.com/?q=XS+speed+sailboard&t=h_&iax=images&ia=images#

iWindsurf Community :: View topic - XS Speed multihull windsurf rig.. anyone know what this is? http://www.iwindsurf.com/forums/viewtopic.php?t=18163

I ran one of these sailboards with a 60 lb thrust Electric trolling motor at a top speed of 6 knots many years ago. The motor was mounted Way Forward from your picture position, between the two hulls at the image crossmember. I don't remember any turbulence at 6 knots, from my viewpoint sitting on the platform.

 

It wont plane because you forgot the hydrofoils!

Too bad you didn't try just cutting a hole for the motor first before cutting the board in half.
Lighter, less wetted area, and better planing surface.

 

I suspect the longboard idea had some disadvantages. So you cut it along the centerline and made a sort of catamaraam. Very well it may have enough stability because of the separated hulls.

The wakes between the hulls crash into one another at some point aft of the stems. If the wakes converge near the lower unit of the engine you have a turbulent mess that does not agree with the propeller or other wet components.



If you want to have a planing hull with that tiny engine there are two things that you must address. The first is to keep the whole structure and the weight of the skipper and other items.........as light as you can make them. The second consideration is the area of the supporting surface. The long board probably does not have sufficient bottom surface for the intended purpose. ......... At a speed of about 10 knots the bottom pressure might be somewhere in the region of 9 pounds per square foot. If the all up weight of the boat is say....200 pounds then you would, theoretically need 200/9 = 22 square feet of bottom. Note that that would be a flat bottom. If the bottom is rounded in section, then the affect of the hydrodynamic pressure is diminished by the escape of some of the pressure toward the sides of the hull. not to confuse the issue here but the flat hard chine bottom also suffers loss of pressure, but not as much as a rounded chine or convex bottom.

You want to have a hull that will plane with a very small engine..................then design a flat bottomed skiff that has sufficient bottom area. I will venture that you can have a small dinghy that weighs no more than the catamaran and does have the needed bottom area.

 

First of all, ventilation is your problem not cavitation which is a very different thing. This little addition to your outboard will make a massive difference: Side Mounted Outboard Motors: Side Mounted Outboard Motors https://outboardfoil.blogspot.com/2019/01/side-mounted-outboard-motors.html

 

I had a plastic vacuum formed Sportyak that planed with a 1.5 hp Johnson. I have no idea how fast it went.

This was many decades ago, I was 10-12 years old I think.

WEST MARINE 7' Sportyak 213 Dinghy | West Marine https://www.westmarine.com/buy/west-marine--7-sportyak-213-dinghy--19948553

 

Key information there, your age, I assume not much weight involved.

 

You young scrawny as$ probably weighted 65 pounds when wet

 

Thank you all for your comments and advice. The stern was digging in rather than lifting out of the water so it could be a trim issue. So firstly I have dropped the outboard a further 4cm and trimmed down about 10 degrees. I doubt if this will be enough to solve the issues but it will probably help, and these are the easiest first measures to take.

I now understand the difference between cavitation and ventilation, and it is clear that I was using the wrong terminology. I have read more on the subject, and I'm sure that I have some kind of ventilation issue. If the measures above don't solve it then it is back to the drawing board. I am interesting in @messabout's comment regarding the design as a whole. Does it actually have enough bottom surface area? Obviously it planed nicely as a windsurfer (even with someone much heavier than me) so I'm fairly sure the extra weight of the deck and engine wouldn't be a problem. I can see that cutting it in half didn't help in this regard, but obviously is would have been hopelessly unstable without doing that. I need to tidy up the inside edges of the hulls which will involve adding an inch or so of foam so nothing that significant.

I'm hoping that a combination of a few different measures will be enough to get it going smoothly. Once I've got a working prototype I shall make it out of stainless brackets and marine plywood, and glass over the inside of the hulls.
I'm also toying with an idea of building a vertical fin type structure to hang down from the deck immediately in front of the engine shaft to act as a streamlining for water flow. It could also have a horizontal plate at the bottom, just below water level, to help prevent turbulent aerated water entering the prop. Can you envisage this or would a drawing help?

One last thought was regarding the prop. I am currently using one with a 5.7inch pitch but they also make a 7" pitch model (same diameter). I was going to switch at some point to gain some extra speed but wonder now if a shorter pitch is contributing to the problem. It says the 7" pitch is designed for lighter boats, and there can't be anything much lighter than this setup, so would a higher pitch prop get more 'purchase' on the water? I would assume either a larger diameter or more blades would be required for this. I've read about pontoon boat owners switching to 4 blade props for similar reasons.

I will keep you all updated with any progress, or at least the outcome of all this.

Any more comments / help would be supper appreciated.
Thank you.

 

Some ideas posted within your message below.

 

Still concerned that I don't have enough surface area for planing but want to try everything else first.

I have read that having a sharp angle between the bottom and the transom is best for a planing hull (it causes the wash to release more easily).

Do you think it would be worth the small loss of buoyancy (not a lot to spare!) to cut off about the last foot aft i.e. a line roughly between the toe strap fixings. It would create a proper i.e. broader planing edge. It would bring the center of the wetted planing area forward. The planing area overall would be less affected by the wash of the outboard. NB as I have said before I did try just moving the outboard about 10 inches aft but that didn't help.

@portacruise - Thank you again for your comments. I understand that a 2.5hp outboard has about 80-90lbs of thrust. I have no idea how that compares with a 5.7sq/m sail in the hands of a very inexperienced 75kg windsurfer! The sail probably creates a significant amount of lift as well.
Would love to try a 7" pitch prop.

 

I have run the same motor on a similar size and weight catamaran. The problem is ventilation of the prop. You need to put a wedge-shaped skeg in front of the OB leg that is about 3/4 as wide as the leg and has a very narrow angle. And it needs to basically seal against the leg when the leg is down. Prototype this with a 1 1/2 board cut to a wedge to get the trim angle correct. With a bit of mild prop work, I could hit 20 knots solo, and could plane about 500 pounds at 8 knots or so. This was my cruising dinghy. (I think I was running an Evenrude 4hp for 20 knots)

See here for another thread - Outboard motor transom design https://www.boatdesign.net/threads/outboard-motor-transom-design.46725/

 

Thank you for this advice, it turned out to be spot on. I have made exactly what you said. I took the boat out today and it stopped all the ventilation issues I had previously.
The only issue (and it may be fundamental to the design) is that the top speed was the same - maxing out at just over 8mph. It didn't quite get onto the plane but it was much smoother. There are still some significant turbulence issues created by the non-smooth inner edge of each hull which I will address in the 'proper' build. There is also interference between the wash and the hulls protruding aft of the engine - thinking of cutting them slightly to finish with a straight edge.
I'm still concerned that there might not be quite enough planing surface to support the boat at such low speed. I think the engine was reaching its rev limit so maybe a prop with a larger pitch will make just enough difference - I can swap the 5.7in prop out for a 7in. Any one have any idea what the maximum speed (allowing for slippage) is for a 2.5hp outboard driving the standard 5.7in prop?. If those last two tweaks don't make a difference then I may be limited to a modest cruising speed with these hulls which would be a shame.
The thread above was extremely useful as well - cheers.

 

What are the hulls made out of ? You should be able to make yourself a new hull using 3" think 2lb extruded foam and you could make it longer and wider (than the original 1 piece long board) eliminating the gap. You should be able to do that for a lot less than 3okg even with fiberglassing. Sounds like a helluva heavy longboard to me.... My wife has an 18' kayak that is fiberglass over plywood in stitch and glue and that is close to 60lb for a nearly completely closed structure.

Just checked, a sheet of 1.5lb density foam 3"x4'x8' is 11.8 lb so lots of weight budget left for such a simple shape which would be pointy on one and anyway...