Repowering an 18 foot Sunbird cuddy

[lander12]

Hello to the group and good day. I have an 18 foot Sunbird cuddy with a 2000 V4 115 hp Yamaha 2 stroke using premix fuel. My plan is to use the boat for fairly short deep sea cruises with an overnight stay in port. Longer stays are probably not practical due to the cramped quarters aboard.

The outboard is not very fuel efficient (some posters have already commented on this) and I am thinking about repowering the boat with a smaller (?60 or 70 hp) motor to increase the boat's range without re-fueling. Having to mix fuel and oil while in port would also require me to have to carry an extra Jerry can to mix the fuel/oil before putting it in the tank.

The boat's on-board tank holds 20 US gallons. I would prefer to make the round trip without re-fueling. I prefer to stay with Yamaha. The boat is rated for 110 hp maximum and probably left the factory with a 90 hp motor.

My question is- how much smaller an outboard can I consider before the outboard/boat combo becomes unworkable? I only need reasonable performance and would not be running the outboard in a full bore situation.

Any thoughts/suggestions from more knowledgeable posters welcomed. Thanks in advance.

 

[aliboy]

Firstly, if that is a carb'd 115, the Yamaha version (SWS2)of that engine is horribly hungry on fuel for it's size. No known reason, and Yamaha won't comment as far as I know, but I know of quite a few SWS2 115hp engine owners who have spent silly amounts of $'s trying to make those engines more efficient. Most of it with Yamaha techs who probably already new that it is a design issue of some sort. The good news is that you should see significant gains in fuel use from an engine change.

Lowering the hp to 60 or 70 will not improve your fuel burn over say a 90hp engine unless the low hp makes you run slower. At the end of the day it takes a certain amount of hp to shift your boat at a given speed, and we are only talking about maybe a small amount of weight difference in the engines and whether they are running in an efficient part of their power band at the speeds you want to travel. Underpowering a boat can actually cost you in fuel when the boat can't maintain a steady plane in a rough sea, or when the engine is running too hard at the speed you want to travel. My guess is that a 90hp engine would be about right for a 18ft cuddy boat. If you went to a 90hp 4 stroke engine (check weights for your transom though) it would probably come close to halving your fuel burn if the current engine is the carb'd model as mentioned above. Personally I like the Honda at 90hp (we have one in our fleet) and from a specification and reputation point of view the 90hp Suzuki would be worth looking at as well.

 

[lander12]

@Aliboy- many thanks- this is the information I was looking for. Yes, indeed, my Yamaha is the carb version and I discovered early on in my ownership of this unit that there was a fuel efficiency issue with it and the same issue was debated at some length on the forums. I had wanted to get an idea of where different horsepower units placed in the performance versus fuel consumption curve. I agree with you inasmuch as underpowering the boat would create as many issues as overpowering it and indeed, probably more over the longer term. Due to my limited budget though, I probably will be forced to stay with a 2 stroke unit but will be looking for an oil injected unit rather than my present pre-mix.