Pacific catamaran outboard conversion

[WillyOcean]

Hey folks,

I saved a 18’ pacific catamaran from the dump. It was too far gone to keep it under sailing power Plans are to fit it with a small outboard to putt around and snorkel and fish. Hopefully be able to tow kids on boogie boards.
I’m new to the site. After finding the site there is small talk about this being done but I’m unable to find anyone who actually finished one. I heard of Owen’s fitting outboards but can’t find any solid information.
So far I have fixed up the holes and weak spots and glassing the seam of the hulls to the super structure. To stiffen it up.
if anyone has any info on this conversion it would be much appreciated. I have lots of ideas and things I hope to avoid but I’m not a boat builder or engineer so any help would be much appreciated

 

[bajansailor]

Welcome to the Forum Willy.

Do you have any photos that you can post of your Pacific catamaran please?

And where is 'Carnation' - I am imagining a little island in the pacific somewhere?

 

[WillyOcean]

I live full time in Washington but grew up in Hawaii and still have family there. Travel back as much as possible
I attempted to up load photo will see if I can get it to work.
towed it to a boat launch got it in the trailer Stripped the left over sailing parts that where not already scavenged
In pictures you can see my buddy’s pact that he put an out board on. He just glassed in a transom off the back of the platform. I’m planning to take it farther in hopes to gain control and performance.

I would like to make a transom between the hulls farther back. the dream is to add a small poling platform above the motor and casting area between the hulls up front. Make a flats cat. Also troll and but and snorkel and pull boogie boards and nieces and nephews

 

[WillyOcean]

My buddy powered his P-cat with a 6hp. And it gets moving but that leaves a little to be desired especially with the weight I plan to add. At this point I’m toying with the idea of adding more floatation to the back of the hulls to accommodate pushing the outboard back. Added flotation could be used in the area in front of the motor to calm down turbulence so the prop gets good clean water. Would be awesome if some has built one of these and has pictures.

I was originally thinking 10 hp but a 20 hp tohatsu doesn’t weight much more.
Not needing to win any races but need to be able to turn and get moving if needed.

 

[fallguy]

You’ll need to establish the transom waterline and depth of hull before the waterline. You don’t want to add hull downward because that is hook which is very bad.

 

[fallguy]

You can build the yellow all the way to fair, but you cannot build the red.

 

[fallguy]

Another option would be to add a nacelle or center engine pod, like a power trimaran. You’d lose the double engine opportunity, but you could set up the center section like a more conventional outboard hull. Just a thought.

 

[messabout]

It would be good to make some educated guesses about the two wakes between the hulls. At some speed the wakes crash together when the boat is flat. Be sure to put the 0utboard at some point that is not near the intersection of the aft end of the clashing wakes. In sailing trim the boat was heeled most of the time and that influenced the location and severity of the clashing wakes. In sailing trim. not a big problem. In motorized trim it can be a big problem unless the most ideal location for the prop and leg of the outboard is selected. To make this more complicated, the critical location moves with different speeds.

It is not out of the question to consider offsetting the motor from center as an option.

 

[fallguy]

The boat will most likely be limited to hull speed. I’d say a 10-20hp motor will be maxed out.

But wiser men may be able to determine max horsepower for a known length; could be 15hp.

 

[WillyOcean]

Im thinking center single motor and like the idea of a center engine pod that connects back to the deck.

 

[bajansailor]

The boat will most likely be limited to hull speed.

And the 'conventional' formula for 'hull speed' (approx 1.3 x the square root of the waterline length) is for hulls with a 'conventional' length / beam ratio of approx 3.
Willy's cat hulls probably have a L/B ratio of 20 (or more), so they will not subscribe to this rule.
The cat in my avatar photo has displacement hulls (ie not planing), with a WL length of about 45', and the L/B ratio at the waterline is about 15; we have had her doing 25 knots flat out with 2 x 150 hp, but I don't think she can do anymore than that, even with more power.

 

[WillyOcean]

Appreciate your input Fall guy and messabout
Bajansalior I have seem some of your posts on other threads. I’m going to look into this. Not looking for super top speeds.

Fall guy I very much appreciate your advice but I don’t under stand your drawing I looked up the term hook referring to boat design on the www and I think your talking about the shape of the bottom of the hull. Is that right?
Is the red line the hull?
What the green line?

I feel like there are two people for a project like this. One who is smart and knows what they are doing and one who is dumb enough to try. I’m just smart enough to know I’m not the first one.

 

[WillyOcean]

Another option would be to add a nacelle or center engine pod, like a power trimaran. You’d lose the double engine opportunity, but you could set up the center section like a more conventional outboard hull. Just a thought.

I think this is what I’m shooting for. I may start looking for a design to build one or find section of a cat hull on the way to the dump and glass it in starting somewhere under the deck extending back and also connecting the hulls and build the transom off it.

 

[WillyOcean]

You can build the yellow all the way to fair, but you cannot build the red.

View attachment 206410

FallGuy I now think I understand you drawing
green is the hull shape now
yellow is what you suggest and I agree with
red is no bueno

 

[fallguy]

FallGuy I now think I understand you drawing
green is the hull shape now
yellow is what you suggest and I agree with
red is no bueno

Right. If you extend the hull or make it deeper and not fair to the rest of the hull; the hook would be enormous and it is really very bad.

As for adding another hull section for an engine; it is not super simple to get a piece from the dump because it will need a 12-13 degree transom added and it will need to be bigger, unless you just build a nacelle, but even that would be pretty custom. Probably need to plan to build something.