Outboard size question.

YeahJohn · #1 (**permalink**) 02-10-2011

I have done tons of research and am now simply looking for opinion. My boat is 27' has a 5375 pound displacement, and currently runs on a 15 hp 2 stroke 20". I normally run it about half throttle 5 knots. There is a prevailing wind and current I have to come in against regardless, going to my slip. I sometimes have to give it about 3/4 throttle to make it up the channel at 4 knots. I have a new garelick 4 stroke bracket and the transom seems solid. Would a new 9.8 tohatsu 25" 4 stroke be enough engine? Everyone says 4 strokes have more low end drive than 2 strokes but have you ever downsized a engine? Thanks.

John

schocksantana228 · #2 (**permalink**) 02-10-2011

the 9.8 will be lots

sailingdog · #3 (**permalink**) 02-10-2011

The one thing you may have to do to get the best performance out of that engine is to get a different prop. Most outboards are not propped properly for a displacement boat...

travlineasy · #4 (**permalink**) 02-10-2011

Much of this will depend upon wind and tidal conditions in the area you intend to sail. You may want to contact the manufacturer. Most have engine specifications for every boat made and what engine will be the most efficient for any vessel.

Good Luck,

Gary

LakeSuperiorGeezer · #5 (**permalink**) 02-10-2011

Well, 9.8 horsepower gets you to a speed of 6.1 knots. The hull speed is 6.4 knots with a length of water line of 22.92 feet. The propeller size choices run 6.5" - 9.5" which is quite a range. The gear ratio is 2:08 for all shaft lengths

SlowButSteady · #6 (**permalink**) 02-10-2011

(Edit: Oops! hehehe...got that bass-ackwards)

I agree with schocksantana, a 10 hp outboard should be more than enough for that boat. A ~~higher~~ lower pitched prop might be a good idea, but be careful. It's easy to "over-pitch" the prop on little outboards, cause the motor to lug, and damage the motor over the long term. Long-shaft outboards are often (usually?) sold with a ~~higher~~ lower pitched prop since they are usually intended for sailboats or as a "kicker" for larger powerboats.

LakeSuperiorGeezer · #7 (**permalink**) 02-10-2011

The Tohatsu 4 cycle 9.8 horsepower MF S9.8 outboard with ultra long shaft high thrust has an 8.7 inch diameter propeller with a 5 inch pitch and 4 blades. This is probably the propeller you need, but it has to be checked with a tachometer because you must be in the 5000 to 6000 rpm range. It is essential not to exceed 6000 rpm as this can damage the engine. Part number is 3B2W645110 for this propeller. More options here: Tohatsu Propeller Chart for various engines.

YeahJohn · #8 (**permalink**) 02-11-2011

I wouldn't mind going with a 15hp tohatsu but it only has the 20" shaft. Plus I am concerned about the weight of a 15hp 4 stroke. We motor to Catalina island some when there is not enough wind to sail over and immediately out of our harbor is practically open ocean. I would hate to be under powered but I would also hate to overkill the engine. Thanks for the input.

CoralReefer · #9 (**permalink**) 02-11-2011

I have a 26' with about the same specs as yours. I have a 15hp 4 stroke Honda with a power prop. The current in the intercoastal here in Fl can reach up to 2 knots and going through the inlets can reach up to 4. I appreciate the extra power when I hit currents.

dabnis · #10 (**permalink**) 02-11-2011

Yea John,

I had a Coronado 25, about 4,500 lbs, IIRC, with a
15HP 2stroke. Ran a prop with 1" or so less pitch
than stock. It would drive the boat as fast as it
would go through any chop and headwind we wanted
to be out in, plus some. Prop configuration and shaft length are vital to "getting the power to the ground".
Anything over 3/4 throttle in calm water just made
the stern squat more. However, when rough and
against the tide, power is good!!

Dabnis