Homebuilt Personal Watercraft

Fifty or more years ago before the advent of jet based personal watercraft
I seem to recall all kinds of plans in magazines such as Popular Mechanics for
tiny run-abouts and tunnel hull inboards and outboards that you would build
from plans. I have not followed real well the evolution of PWC personal watercraft
but the ones I have seen lately have a lot more hp than all the boats my family
had growing up that we would water ski behind and zoomed all around the lake in.
That and the fact that I remember as a ten or twelve year old I even
skied behind my grandpa's fishing boat that had only 35 hp and it was just
me an him on the lake. So I am therefore led to believe that the jet
drives are MUCH less efficient and probably became popular because they
did not want kids swimming around small boats with props exposed to
sever limbs etc. Today our little industrial engines from Briggs and Honda
have made great strides in that the largest V-Twins are up in the low to mid
30 hp ranges now and I think a small little wooden boat with an inboard
lawn mower engine could be designed ( redesigned ) to again zoom around
a lake at least 35 or 40 mph or more. The other great benefit would be
the fuel savings for a whole day at the lake of using 35 hp instead of 150hp
as exists on a lot of the PWC.
I have actually found a few pics on the old POP MEchanics mags etc. and will try to attach a few.

Also I went to a few of the big boat plans websites and found a few 11 and 12 foot
plans for homebuilt inboard boats. One boat was even smaller and would get up
onto plane with one person while using only a ten HP outboard motor.
18mph I think they said it could go. not bad for just ten HP.

 

Making a garden tractor engine work in a boat is more trouble than you might realize, assuming you'd prefer it didn't blow up on you, underway. I've done this a number of times (used small gas engines), but some marinization needs to be preformed and frankly, these engines aren't the best suited for this type of work.

It's usually just cheaper (much) to find a used outboard, fix whatever ills it might have and go have fun, than to convert a lawn mower engine. You see the engine is just part of the problem. You'll need a transmission and if it's a simple straight shaft, you'll need a rudder too. Rudder steered boats steer like crap, BTW. So, you have to ask yourself, what advantages will I get if I go the homemade drive route, compaired to an outboard or jet.

 

The blowing up part came to mind readily. PWC are highly evolved these days, and those old mini-boats with outboards would have chopped up a few swimmers back in the day.

 

Yes, see http://www.youtube.com/watch?v=BzU_22jASoU . The VERY hard part is at 2:31 = Restarting the engine AFTER being capsized.

 

The major issues are fuel vapor collection inside enclosed spaces, heat build up (big issue), how to handle the exhaust and of course eliminating spark production, within the enclosed engine space. Then you'll need to figure out how to shift FNR on a transmission, how to setup a throttle, remote wiring for the starter, charging circuit, maybe some gauges to monitor the engine. This doesn't address the steering concerns, just the basics for the drive system. Yeah, it's a bit more complicated than first glance might suggest.

 

If that lot doesn't convince him it is a dud idea, nothing will !

 

Smith Island Crab Skiffs use 20 HP air cooled inboard engines and are raced.

https://www.facebook.com/Smith-Island-Crab-Skiff-Association-295269101404/
http://www.woodenboat.com/boat-plans-kits/smith-island-crab-skiff
https://www.youtube.com/watch?v=PtcgXa4XIqg

 

Good points about the requirements to put an alternate engine on a boat.
Makes a nice check list for those who want to try.

But how about letting the guy try. Something simple and useful might come of it.

Every useful idea started someplace.

 

Before you go off building a mini speed boat and spending money,I humbly suggest you take one out in water that is not smooth.

 

What ! Not a single comment about the photos I attached. Are all
you guys heavily invested in PWC companies or what? With the price of
PWC up in the 20K and 30K thousands of dollars.... how can there not be
more interest in smallish homebuilt boats ? I also liked those teeny tiny tunnel
hull mini racing boats but perhaps they are using small outboard motors.

Thanks thanks thanks for the Crab Skiff links.... Thats what I am talking
about but I cant find any technical info about the boats other than you can
buy the plans for the wooden boat.

I did also see a reference to 20 hp Vanguard as the motor type on those
Crab Skiffs so I would think an upgrade to 35 hp might be about as much as we
could stand.

 

There are many Glen-L builders online. Glen-L is the largest source for these plans https://www.boatdesigns.com/Outboards/departments/10/

 

Plenty available used.

 

I can't see much point in putting a 20 or 30 hp engine into a tiny little thing that has little practical use, you'd be bored with it after doing a couple of figure-eights. And as had been pointed out, will bash you around in any chop. Might as well put it on a 12 or 13 foot "proper" boat.

 

Where? I am not seeing them.

I perused through the Glen-L website and they have a wonderful selection
except they really dont have any small boats for 20hp inboards.

Full Disclosure here.... I'd love to work with an outboard motor as suggested
but I have been given a 25hp Kohler VTwin and the little Crab Skiffs seemed
to scoot along nicely on their 20hp inboards. And what about those little
hydroplane racers ? Are they all using outboards?

Maybe it is just difficult( or expensive) to piece together the hardware
necessary to put a shaft drive through the hull with bearings and waterproofing etc.
for the real inboard function. I have no idea where to start and I do
not want to reinvent from scratch. There must be some smallish inboards being built and
enjoyed somewhere that they have worked out all the pieces and parts necessary.

 

"crab skiffs" ? Are these boats used professionally ? Seems to me that if they can't afford a "real" marine engine, the game is a crock !