Outboard Sizing

Good day members.

I have a Matelot 20 hull, picture attached, which I have converted to outboard configuration to take a long shaft outboard on the transom.

She is 20.3ft LOA, 17.8ft LWL and weighed 1.23 tons on build, however this was with a 8hp Saab Inboard and gearbox on board. It is a completely displacement hull and will not plane, I believe her hull speed to be 6 knots.

My question is what size outboard should I use. I use the boat for coastal fishing in waters with a tidal stream of up to 3 knots.

Thank you in advance for the advice!

Regards, Chris.

 

I think I'd opt for a 9.9 with the extra long leg, and gear ratio and prop pitch suited to the low speed.

 

10 hp sounds about right. Choose the engine with the highest dc charging output so your ships batteries stay charged . I dont know the 10 hp class motors. If you will be using remote controls be sure that your chosen motor can handle them .

Models marketed as HIGH THRUST might be worthwhile. Extra long shaft motors would be worthwhile

 

5 HP will push this boat to displacement speed easily enough, though a reserve margin will be fairly small. An 8 HP would be more than enough with sufficient reserve for chop and contrary currents/winds. 10 HP would likely be a bit much and you'll never be operating in the engines preferred range, unless you're in severe use areas with a hefty load. What ever engine you decide to get, pick the "high thrust" model with a long shaft.

 

We have a sailboat, 20' LOA, around 19' LWL, 7' beam overall, around 1.6 tons displacement. A 6 HP Mercury outboard at around 3/4 throttle will push it a 5 knots on a calm day. Full throttle perhaps another half knot.

 

I have a similarly sized boat and it's 5 HP high thrust has had no issues. FNR, long shaft and high thrust are the key features for a boat like that, I would think.

 

Ditto all of the above. What you are searching for is the lowest hp rating for a given block size or engine displacement. So if one mfg has a block with a 6-10 hp series and another has a 10 -20 hp range, you want the 10 from the 10 - 20, not the 10 from the 6-10. It will be the one that comes with the right prop pitch and diameter for your use. The guy with the 200# plywood hydroplane that does 60 mph would do the opposite.

 

I think I'd prefer the inboard, but if it must be an outboard, the height of the mounting is important, you want sufficient immersion so the prop stays in the water, but not so much that the unstreamlined part of the leg is continually dragging. That should influence the choice of engine, and the transom ledge height..

 

Didn't we go through all this a while back. Why don't you stick to one thread so people can read all the posts. I still think the 9.9 is a good choice.

 

Thank you very much for your advice guys, it is much appreciated. I have been offered a mariner sailmate 8hp engine, 2 stroke version, I think this engine may suit the boat being a high thrust version.

Apologies if there was any confusion relating to my previous post, this was relating to the conversion of the hull from inboard to outboard configuration. As I made my decision and converted the hull, I believed this was a new topic.

Thanks again for the advice!

 

The 8 hp sail version will be excellent because it has the reduction for pushing heavier loads than a standard outboard.

 

Unfortunately the 8hp saildrive was snapped up before I got my hands on it! What I have been offered is a new mariner 9.9hp 4 stroke, it is a long shaft outboard but isn't a high thrust or sail drive version. Any thoughts? Thanks for the advice!

 

No good, you want a motor tailored for low-speed thrust.

 

Agreed, you could make the 9.9 work, but it'll just beat the water to a froth, with it's shorter gearing, dramatically decreasing efficiency and durability.

 

My Achilles 24 yacht, 1,200kgs with a hull speed of 5 knots needs 6hp but 8hp provides reserve when against the tide, wind or waves. 10hp would be more than enough.