Outboard HP/Weight

[Plymouth Sailor]

Can anyone comment on the limit of HP/weight I can place on my Hunter 1976 25?
I'm finding more and more that power on a sailboat counts. I currently have a 8hp reliable outboard however I'm thinking of someday upgrading to a 25hp motor. I've been told that an outboard too heavy could tear the transom off the boat. I currently have a special factory order inboard that has no compression and will cost $3000 to fix so I need more power to push this inboard and the remainder of the weight on my boat. Any knowledge/wisdom appreciated.

Thanks!

 

[Justin_NSA]

Keep what you have. It's perfectly adequate.

 

[braol]

I don't know about a maximum, but I have only an old 4 hp outboard on my H27 and it pushes the boat along just fine. Maybe not against a current or storm, but it gets me in and out of the harbor just fine. Only problem with the outboard is popping out of the water with any kind of good wave action.

 

[Plymouth Sailor]

I've been sailing since 1976 where I cut my teeth on a home made Glen-L 16' catamaran and we didn't even have a motor but I feel with a 5000 lbs boat particularly in a strong wind against me, power is preferable to tacking when the tide is going out and the sun down ( I've never owned a "stinkpot"). I'm seriously thinking of that 25hp 4 stroke!

 

[kito]

I'm not sure what you are expecting by putting such a large outboard on it.

 

[Justin_NSA]

My Hunter is 11,000 lbs displacement and I only have an 18hp diesel.

 

[kito]

Mine is 9700 lbs with a 10 hp diesel. For the price of half that 25 hp 4 stroke you can get your diesel fixed. Also, I imagine a 25 hp 4 stroke is a lot of weight to be hanging off your transom.

 

[Plymouth Sailor]

Stronger resistance to tides/currents, longer trips, tougher weather...

 

[Plymouth Sailor]

Repair work on the inboard is difficult at best as it is required that I remove the engine for a technician to correct the compression issues, but thanks for all your comments!

 

[Benny17441]

I'm confused abut the inboard as it relates to the outboard. I guess your question boils down to, how much weight will your transom support? I would not know as the boat is likely 30+ years old and I do not know its condition but in general terms with an adequate motor mount it should be able to support a 9.9 four stroke outboard weighing around 100 lbs. Understand that boat induced motion will impart acceleration forces to the engine perhaps double their load on the transom. The h25 Cherubini is a displacement hull which means its theorical top speed is limited by the length of the wetted surface. All these is physics talk for raw calculations that indicate your boat with a waterline length of 20' 2" has a theorical maximum speed of 6.2 knots. This theorical speed can be exceeded but it requires ever increasing amounts of horsepower to deliver small gains. You can probably attain that speed with your 8HP and any increases in HP may only result in extra costs and fuel consumption. I doubt very much that you would need or significantly benefit from a 25HP engine. These calculations would lead us to believe that the best engine for you would be the smallest engine that got you to that hull speed but things are not that simple. An outboard with more horsepower can accelerate quicker and push better against opposing currents and wind and provide an added degree of safety. While 6 HP might do for calm lake waters perhaps 10HP are need for a river with strong currents. The most important concern about the engine is that it be equipped with the proper propeller size for the boat. Sizing a propeller can be more of an art than a science as it involves the hull shape. What you are looking is for a prop that allows the engine just to reach maximum rated RPM at wide open throttle. This will insure maximum speed with adequate acceleration. A prop that could allow the engine to rev in excess will not attain maximum speed and could damage the engine. When using an outboard to power a sailboat it is recommended you get one with a 20" or better shaft length. This will minimize the propeller coming out of the water as a result of wakes or surface swells. In a calm lake it might not be an issue. As a last issue look at the engine features some 9.9HP engines may come with tilt/trim, electric starter, and alternator while your 8HP may lack all these. If you have the budget those might be a reason to upgrade. An increase in HP adds cost, fuel consumption and weight; just get what it is necessary for the prevalent conditions at your location. Power in a sailboat only counts when you are trying to run away from a storm, trying to get up river against a current of 3+ knots or pushing against strong winds and currents. For the rest of the time the engine is just an auxiliary.

 

[Plymouth Sailor]

Thank you for your detailed knowledge! I'm on the East Coast with ocean conditions, and yes, the 8hp is fine which I usually don't run at full capacity however I'm concerned that I'll blow it out being that it's pushing a proportionately heavy boat compared to a skiff with the same engine.

 

[braol]

You should be able to (with a little time and some dirty hands) replace the head gasket. Mark all your diesel lines and don't loose the little washers. Bleed diesel lines after hook-up and there you go. Gasket is about $40. This is more of a time-consuming job than a difficult job. (I actually thought about going full-on outboard and removing the diesel to gain space...but that's another story for another day...

 

[Plymouth Sailor]

So that would solve the compression problem? I thought I would need to have a full valve job done?

 

[Benny17441]

Thank you for your detailed knowledge! I'm on the East Coast with ocean conditions, and yes, the 8hp is fine which I usually don't run at full capacity however I'm concerned that I'll blow it out being that it's pushing a proportionately heavy boat compared to a skiff with the same engine.

A sailboat is the perfect Hybrid, if the engine goes you have the sails. Your engine does not know it is pushing a sailboat or a skiff except perhaps for the different prop requirements for each boat. Your sailboat may weigh 5000# but that weight is supported by the water it displaces; it takes very little effort to get it moving. Whenever I go out in the sailboat the least of my worries is loosing a source of power, it would be rare that you would loose both the sails and the engine at the same time. It did happen to me once on a trip just two days apart but for those occasions tow BoatUS insurance brings a third source of power.

 

[frankies56]

8 hp is quite enough. sailboats are not made to go fast. outboards are very reliable, if your looking for speed buy a speed boat. 25 hp is to much and you will not go any faster just use more fuel. what is important is having the right prop pitch to give you the torq you need. and the right rpm the eng needs not bogging down or over reving. I think the only problem is when rolling waves are comming at you as the boat pitches the motor tends to come out of the water. it is a little frustrating but manageable. inboards are a pain to work on to and the prop tends to foul if not cleaned often. but I have a 17000 pound boat and it has a 33 hp yanmar. it gets me to hull speed at the right rpm if I push it it wont go any faster. my best

 

[Alctel]

25HP is way overpowered for a 25' - as said it won't go any faster (unless against strong winds - note that it WON'T help against current) as the maximum speed the hull can go through the water is fixed no matter how big an engine on it (unless you get it to plane or something)

My 36 still has the 20HP 2QM20, for reference.

 

[Siamese]

It'd be terrific if all your engine needed was a new head gasket. It'd also be unlikely. If it's the valves, then that would be pretty good too, as you'd remove the head, have it reconditioned, and bolt it back on.

If it's your piston rings, which is a likely scenario, then yes, you'd be pulling that engine and dropping a fair amount of money into it.

If you do an outboard, you'll likely want to reinforce the transom. Definitely no to a 25. I wouldn't exceed 10.