Amateur needs guidance. Experimental Build

Me and my partner have been looking to install a small 11 hp horizontal shaft engine with a foreward/reverse gearbox in a small 150lb 12' wood dory. We were looking to run an inboard set up (gearbox / shaft through the stuffing box / to a prop). Assuming our top end RPM from the engine is 2500rpm - 3000rpm and the gear box were looking at is 1:1 forward, what dimensions on a prop should we be looking for? Not sure on clearance yet for the prop but i assume we will be able to fit 4-9 inches. We have not bought anything yet and the engine we have our eye on is a very cheap 4 cycle (they do not list torque or rpm specs. They just say 346cc). Any advice on what we will need in torque?? What can we expect from 1:1 forward ratio? We expect to use a clutch.

Please keep in mind we are amateurs that seek knowledge.
Any info will help, and please ask any info i have not added.

Thank You

 

Hi AVBoater,

The answer will depend on the hull shape of your dory and on the total weight of the boat. Assuming a non-planing boat, some 800 lbs full weight, you can imo expect a max speed between 4.5 and 5 kts. A 4-5 HP motor should be enough to move the boat at that speed, all the rest of the power will go into aerating the water and heating the planet. Plus, it will be a dead weight and wasted space on board.
Hence, if it is a non-planing dory, my advice is to opt for a smaller engine. If you still want to use the engine you have, bear in mind that it will be working at continuous 40% power output, with all the related issues regarding the fuel efficiency and engine life.

Regarding a propeller, and again assuming a displacement dory, the 1:1 ratio will give you either a very poor and inefficient prop performance or an inefficiently working engine at the above mentioned speed. You'll need to reduce the rpm by at least a factor of 2.5 .

Cheers

 

One answer to the gear ratio problem is to use a hydrostatic transmission from a discarded lawn tractor. These will be belt driven which will allow isolation of the engine on rubber mounting for quieter operation. A prop of the largest diameter that can be reasonably fitted will be best for displacement speed. Pitch should probably chosen to give about 25% slip at the desired speed. Don't forget that you need some kind of thrust bearing to take the axial thrust of the prop that your engine is not designed to handle.

 

I assume by 12' you mean a dory which is 12 foot length overall. Does it have a traditional dory shape with a narrow bottom and sides with lots of flare?

 

My opinion would be you don't need gear reduction if the motor is only turning 2500 to 3000 RPM. An outboard motor usually runs up to about 6000 RPM and has a gear reduction of around 1.5 or 1.6 depending what make it is and the prop is designed for the HP and RPM, the manufacturer has no idea of what type of boat it will be put on. So what you have to do is match the prop to the HP and RPM. There is a free program available from Castle Marine LTD. That can calculate the diameter, pitch and DAR of a prop that is needed when put in the HP ,RPM and gear ratio. This program doesn't work with windows 7 I have it on one of my windows XP computers. If I can find time later today I'll run the figures you have given and see what it recommends. I built an outboard using an 11 HP Briggs motor and a 12.9 horse Chrysler outboard leg the prop was too small for the RPM of Briggs. You definitely will need a thrust bearing of some type. What type of machining and equipment capabilities do you have?

Dave T

 

I seem to recall a company that sells plans on powering your small craft with inboard lawnmower engines. Check it out on the internet and this should more than give you all the info you need

 

I ran the specs you gave through the Castle Marine prop calc program. It shows you need an 8.05 diameter with a 4.57 pitch 3 blade prop. The specs I gave it were 11 HP 3000 RPM 1 to 1 gear ratio 12' displacement hull 1000 lbs. It estimates 9 shaft HP .57 slip ratio, maximum speed 4.85 kts 5.58 mph the speed naturally depends a lot on the shape of the hull the weight of the boat and if the boat planes.

Dave T

 

Ok did a quick search and suggest you check out oldmarineengine.com . also google Reo inboard marine kit. The photo of this kit will give you an idea of whats involved -not exactly something a novice would want to build as a back yard project. I also noticed there were a few of these Reo kits listed on e bay. My suggestion grab one if you can it will be the less costly route. As a point of interest the 19ft. traditional dory shown on my logo is one in which i installed a similar setup using an old one lunger (piston) make and break marine gas engine.

 

Thank you all for replying to questions! Very much appreciated. I now have some more questions; how can i tell if my boat is capable of planing. I would like to understand this for my next build. The boat is flat bottom and only has a 2-3 inch rocker. Does a boat need a V hull to plane? More questions to come when i get home this evening. Again I much appreciate everyone taking the time to reply.
-AVboater

 

If the small 150lb 12' wood dory has a traditional dory shape, then an 11 HP engine will be about 5 times more power than useful in the boat. Also with the engine installed there won't be a lot of space for people.

 

How wide is the bottom in the middle of the boat, and how wide at the stern? Also how long is the bottom?

 

An 11hp is a bit big. With the standard lawnmower engine, a typical gearbox drive train setup are two rubber tires. No coupling.

the propshaft terminates at the engine with a rubber tire.

The output shaft of the motor is fit with a rubber tire.

The engine is hinged .

For neutral the engine is lifted , no tire contact...to go forward the engine is lowered...rubber tires contact...CHIRP, puff of smoke...prop shaft turns.

The change the prop shaft rotation speed.. ratio 2 to 1 or whatever.. different diameter tires are used .

might work for an 11hp...bit heavy

 

Michael, that's a very original idea, I have to say... And a pretty smelly one too.
Cheers

 

From what he described earlier I'm guessing he has a regular gearbox transmission not a drive that uses tires. A picture of the motor and gearbox would help.

AVboater if the boat has a flat bottom it will plane if you have enough HP and it doesn't weigh too much how it will handle on plane will depend the shape of the hull and several other factors . With the same specs as before a semi planing boat with 11 horse should do just over 15 mph at 1000# displacement with the above mentioned prop. What is your best estimate of weight of the boat fully loaded with passengers, fishing gear, gasoline and full beer cooler etc. If you could post a picture of the boat, motor and gear box it would really help. My boat is 12' at the waterline and I have a 30 HP outboard on it now and am hoping to have a 65 HP this summer it's top speed so far has been 26 mph if you want to check it out take a look at my thread [A boat a motor and a trailer] under boat building on this site

Dave T

 

That Reo Inboard Marine Kit setup is exactly what he needs. It's the first time I have actually seem the display model. It lays out the whole project as it should be constructed and installed. If nothing else it can be used as a set of plans and parts involved for the DIYer.