Gyula wanted to fit an outboard either gas or an electric trolling motor to the first European OzRacer and wanted to compare power outputs
Fitting an outboard to an OzRacer or other sailing dinghy
He just emailed me asking about fitting an outboard motor to his OzRacer/OzPDRacer (as it was then).
Hi Michael
Can you give me some ideas, how to strenghten the back of my boat so I can put a small outboard on it?
Regards
Gyula
Structural considerations of fitting an outboard to a plywood dinghy
There are a couple of things to consider.
First is you cannot make these boats go fast under outboard. They just stick their noses up in the air and refuse to go faster – while you vibrate and shake them unnecessarily with the excess load.
A couple of horsepower is ideal. You could use a 3 or 4hp as long as you never push it to full revs. Just keep the boat powered up so the boat sits happily near its designed trim.
Second is that this setup is for the OzRacer and OzGoose which have sidedecks to brace the transom. If you don’t have side decks you will need to fit triangular plywood knees across the corners of the boat. They should have a minimum of 250mm (8″) contact with the transom and about 300mm (12″) contact with the gunwale and be glued and, if not using epoxy glue, probably screwed as well.
The idea is to take into account the types of loads the boat will be subject to without adding excessive weight. The ASSUMPTION is that you won’t use more than 2 or 3hp to put load on the boat.
The Transom Modification – it doesn’t need to be a lot
Boats can be modified to follow this drawing while building … or with a slight modification to the method be fitted afterward .. which is Gyula’s situation – I have notes after the drawing to show you don’t need to cut holes in the side buoyancy tanks!
My reply covered how you don’t need to cut open the buoyancy tanks to increase the depth of the top stiffener to 45mm (1 3/4″) for a boat that is already built..
You only need to increase the depth between the bouyancy tanks faces. So just increase the depth in the visible area of the cockpit. There is no need to change the cleat between the bottom and the transom – that’s already structural and strongly boxed in.
Another nice mod for the Oz centreboard boats is to build a seat that fits down the centreboard slot. It should also rest on the beam across the front of the centrecase and only be about a foot wide. It it is too wide you could break the centrecase … maybe … if you sat over to one side all the way. If you want a full width seat you would have to provide side supports on the buoyancy tanks faces or the side of the hull.
This is Jim Post’s Goose going at the speed we like to see.
Video image can take a moment to load.
Use of an electric trolling motor for small sailboat propulsion
Another option is an electric outboard or trolling motor. I do consider electric propulsion as primary a very immature technology. It might get you somewhere, but might not be able to get you back if the wind and waves come up. So only suitable for small rivers and small lakes. Don’t go further from shore than you can row or paddle back.
Some trolling motors are quite cheap and they work OK on canoes as they don’t need much power to move along well. I am not sure how they would go on a wider flatter boat like a PDR or OzRacer. Would love some feedback on practical experience.
Convert the output of the Electric Trolling Motor to Horsepower
To convert the “pounds thrust” of an electric trolling motor to horsepower is not directly possible – as you need to know the speed the boat will do at full power.
It is easier to work out the power in watts simply by multiplying the voltage of the battery by the maximum amps the trolling motor will draw.
Volts x Amps = Watts then divide watts by 746 to get horsepower.
I found on a website that for one model of MinnKota the maximum current draw was 50amps on a 12 volt system.
12 x 50 = 600. Then 600 divided by 746 is about 0.8 horsepower.
Electric Trolling Motors for sailing dinghies – range and battery care
There are two important notes when calculating the power like this.
1/The trolling motors are not meant to run all day at full revolutions so you don’t let the motor run at full revs all day or it will overheat or otherwise be damaged. The more you pay for one the longer you can run it for at higher revolutions. So if you buy one for $100 don’t run it too hard.
2/You can’t run the battery flat. Or … again … the more you pay for the battery the flatter you can run it. Never discharge the battery so it is completely flat.
From wikipedia
A deep-cycle battery is designed to discharge between 50% and 80% depending on the manufacturer and construction of the battery. Although these batteries can be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge,[1] as there is a direct correlation between depth of discharge on the battery and the number of charge and discharge cycles it can perform.