Outboard engine position on power cat

Hello there,

I am repowering a 23 foot custom built, semi planing aluminum power catamaran with 2x Suzuki 175 hp engines (it used to have Yamaha 115's which were insufficient).

I am also installing Bennett M80 trim tabs.

Trim tabs and outboards work best when there is a bit of clearance between the outboard leg/cavitation plate/propeller and the trim tab, something that can be difficult to achieve on a catamaran with somewhat narrow hulls. I am therefore contemplating moving the engines a little closer together - which means they will be positioned along the inside of the hulls' keels instead of just in front of the keels/center lines. This would give the trim tabs better working conditions - and minimize cavitation / propellers catching air due to the propellers and trim tabs otherwise being too close to each other.

Do any of you know what the handling consequences would be if I did that?

 

Hard to say without a picture of the transom end, but what clearance will there be at full steering lock ? I would probably try something like Permatrim fitted to the motors, and maybe transom wedges to get more tuck-under, first.

 

That is a nono; you will have disturbed water along the hull inside. This will cause ventilation trouble to a varying degree, depending on hull shape (hull symmetry/asymmetry for instance).

Instead of conventional trim tabs, use the interceptor model instead, they work fine in this position, and normally have better lift/drag ratio than tabs.

 

From how he described it, the hulls are symmetrical. He wants to bring the motors closer together and fit tabs on the outside halves of each hull. Much depends on the shape and width of the demi-hulls at the transom as to what the possibilities could be.

 

Yes, the hulls are symmetrical. See the attached photos. I will be able to move the trim tab about an inch further out, but the inside edge will still be quite close to the outboard & hull center line.

Note that the first photo shows the Suzuki mounted directly onto the hull. The second shows the manually adjustable Slidemaster 10 inch jackplates I will be using, in order to move the outboard backwards so I achieve more clearance between the outboard and the hull (thereby easily accomodating the length of the M8 trim tab, and more).

I looked up the Interceptor tabs, but even the smallest ones are too wide for my boat.
http://www.humphree.com/products/interceptor-trim-tab/standard-interceptor-trim-tab/

I am in fact also fitting Permatrim M9 Under Mount hydrofoils...:
http://www.permatrims.com/M9_Black_Permatrim_Hydrofoil_p/um9blacksuzuki.htm

The reason why I am going for Under Mounts is that I am at some point also considering adding single prop Minn-Kota Riptide Engine Mount 80 trolling motors which will sit on top of the outboard caviation plates.
http://www.minnkotamotors.com/Trolling-Motors/Engine-Mount/Riptide-Engine-Mount/

And then perhaps also changing the manual Slidemaster jackplates to hydraulic Bob' Jackplates instead.
http://www.bobsmachine.com/Action-Series-Jack-Plates-upto-300HP_c4.htm

My thinking is that I could move each outboard about 2 inches closer to the hull - thereby achieving more clearance between the M8 trim tabs and the engines.

Thoughts?

 

Why do You need trim tabs on this cat? Is there any problem to correct? I would just install the engines at CL, and use engine's tabs for trimming.

From the photos, the hull seems significantly narrowed don at chine, seems to be wrong approach, at least not usual. We usually keep it as wide as possible to enable easy passing the hump.

Anyway, I would test the boat prior to installing any additional tabs.

 

No, the hulls don't become narrower or anything as you move from bow to aft, it just looks that way the way the photo has been taken.
In my experience/opinion it's often better to trim a boat via trim tabs (and you often get a little extra speed that way) than via the engines. At least I would like to have both options - engine and trim tab trimming.

Back to my original question: what do you think the consequences would be of moving both engines a couple of inches closer together? I.e. they would each be two inches off the hul center lines.

 

Cats do not like heavy engines that are set back from the transom. They don't have enough effective beam to support it. I don't like the idea of the trim tabs so much, even if they don't interfere with the outboards, they are fairly small and they will generate a lot of drag if you have to over-apply them to get stern lift. It looks like you would have to move the engines inward quite a bit to avoid interaction with the tabs. You might then encounter some cavitation whwn steering hard into a turn. I would consider welding some angle sections to the hulls at chine level to get some extra lift back there. Being an unpainted alloy boat, it is feasible to do that. And you will need to go further in than two inches to avoid interference.

 

Fully agree. But test the boat first!

My 26' cat is balanced so it will cruise at its best with zero trim of engines. Trimming the boat with tabs immediately reduces the performance, but I do 25% trim sometimes for better control on waves.

 

Thanks Alik and Mr. Efficiency for your suggestions; very helpful. I agree that cats don't like heavy engines, but I had to move mine back a bit in order to be able to tilt them fully out of the water. The Suzuki 175's were a bit higher than the Yamaha 115's.
I like the idea of welding on some angle sections. We can easily do that. But for now I will go with your guidance of completing the boat in standard form first and see how it rides, before making modifications.

With Permatrim M9's on both engines I should be able to get good lift I would think. Also, I have moved my heavy battery bank up front in order to help balance the boat out.

I think the boat will be back in the water in 4-6 weeks - I will let you know how I get on. Thanks!

 

What is the width of the individual demi-hulls, at the chine ?

 

I am not with the boat right now so can't measure it, but I know that each of the bottom sections (you've got the keel/CL in the middle) is 290 mm/11.4" wide - so I think the width of the hulls, before they angle upwards, is probably around 540-560 mm/21-22 inches wide.

The boat is built on these designs (with some modification):

http://www.boatdesigns.com/Bear-Cat-Cuddy/products/722/

http://www.boatdesigns.com/Wildcat-E-X-T-Cuddy/products/772/

The width of the boat is 254 cm = 100 inches. It's around 23 feet long.

 

The angled sides don't count for much underway, though they will reduce draft at rest. 21-22 inches is quite slim really, especially if it a heavy boat, which seems to be the case if 2x115 hp is inadequate. And the extra weight of bigger engines, and the set-back compounds the situation. This may not be so easy to get a satisfactory solution for, but if it does not involve making changes that are difficult to reverse, by all means try your current ideas first. A very popular 23' power cat in this country which was first on the scene around 1974-ish (no V6 2-strokes even in those days) had wider demi-hulls than your boat, which were later "fattened" a couple of inches when V6 engines came in, partly because they wanted to get the powerheads higher off the water at rest, but also to get better performance on-plane. Since then we have heavier 4 strokes, and a fashion to set-back engines, both to allegedly gain some efficiency, and to get more cockpit length. I prefer the older idea, of transom hung, as light as possible engines. You may also find you have a porpoising issue as well in some situations. I tend to think your best prospects lie in creating some chine flats welded to the sides, a 90 degree angle will give some down-angle as well, at least two inches wide, four of them, and I am not that keen to have them extended too far toward the bow, where they will only create bow lift and be counter-productive, and firm up the ride.

 

you need to fix the problem of why a 23' cat with twin 175hp is semi displacement
should be doing 50mph+

 

Oh it will certainly plane, and it also did with 2x 115's - I just saw somewhere that somebody called a similar hull shape "semi planing"... I'll let you know how I get on with the 2x 175's.