Is a 9.9 Outboard Enough for a 38 Foot Boat?

[Wanderlust]

A 9.9 hp should be OK most of the time, and you get the advantage of being able to swivel it for sideways thrust and better manouverability. That's the positive part.

But designing a solution for "most of the time" could be a bit dangerous. What happens when you need to bash into a strong wind or a strong tidal channel? I would be surprised if a 9.9 hp would be sufficient.

You said you wanted to mount an outboard like on your current boat. Does this mean you are not constrained to your existing 9.9hp?

Then there is the question of any outboard hanging off the stern of a sailboat. The stern typically has a lot more movement than a power boat, so it won't take much of a sea to have the prop repeatedly spinning in air.

My suggestion is that the outboard solution would be OK for temporary use while you sort out a proper solution. Just be mindful of its limitations.

Then of course some people have no engine and yet manage OK.

[cal40john]

Interesting how rules of thumb change over time.

Cruising Under Sail, Eric Hiscock:

"For normal cruising purposes one horsepower per ton of displacement should be sufficient - Wanderer IV (22 tons) requires no more than 18 horsepower to give her a speed of 5 1/2 knots in quiet weather. But if the yacht is to be capable of making good progress against a headwind and sea, or to reach her maximum hull speed under power (some owners feel this is essential) at least 3 horsepower per ton will be required."


Previously posted was 1 hp per 500 lbs, which is 4 hp per 2000 lbs. Hiscock probably means long tons so about 4.5 hp per ton is the current recommendation.

Don't follow the old rule of thumb if you expect to motor at hull speed directly into 25 knot head winds.

Old rule of thumb doesn't seem to be entirely dead:
Horse Power per ton?

My boat is about 3.3 hp per ton. Works for me.

[conachair]

Past few passages I've used a 15hp Honda outboard on the back of a 33' 10t long keel. Stationary into a bit of breeze and a chop but moves it in flat calm. Costs more in fuel as well. And no alternater for nav lights at night. Not something I'd do by choice.

[Tomm0]

Go for an electric drive inboard!
I don't have one, never seen one actually going, etc, but the idea really appeals.

[JazzyO]

Originally Posted by cal40john

Previously posted was 1 hp per 500 lbs, which is 4 hp per 2000 lbs. Hiscock probably means long tons so about 4.5 hp per ton is the current recommendation.

My boat is about 3.3 hp per ton. Works for me.

Mine is 27bhp for a 38 footer, displacement 6.5 tonnes. Works out as 4.1 bhp per tonne, or 3.76bhp per ton. I think that works OK, but these days people would probably put more in.

I would never use an outboard to maneuver a 8 tonne 38 footer, no matter how much hp it has, especially when shorthanding. But then I'm tight marinas all the time, in other situations you might be able to make it work.


Onno

[JazzyO]

Originally Posted by Tomm0

Go for an electric drive inboard!
I don't have one, never seen one actually going, etc, but the idea really appeals.

I think that will be a very expensive solution, judging by the cost of Torqueedo's. I'm also not sure it works well if you motor a lot. If you don't, then it might work but you still need serious recharging facilities.

I like the fact that my diesel is an onboard generator. With solar panels and a windgen you have 3 ways of getting juice in the batteries. With an electric inboard you only have 2, and you've lost the most powerful charger.


Onno

[Tomm0]

I have an 8hp outboard but I only displace a couple of tonnes.
And the outboard pushes 5A into my battery when the sun is lacking on my panels.

[damianham]

Originally Posted by JazzyO

I think that will be a very expensive solution, judging by the cost of Torqueedo's. I'm also not sure it works well if you motor a lot. If you don't, then it might work but you still need serious recharging facilities.

I like the fact that my diesel is an onboard generator. With solar panels and a windgen you have 3 ways of getting juice in the batteries. With an electric inboard you only have 2, and you've lost the most powerful charger.


Onno

I think the suggestion was to go for electric inboard not torqueedo electric outboard and it seems like a very sensible solution when faced with the need to install or replace an inboard engine.

As I understand it the electric motor power output requirements for a yacht are less than what would be required for a diesel engine because you have maximum thrust at 1rpm and can use larger diameter props at lower revs for increased trust.

Given OP boat is ~8.5 ton @ 2HP per ton == 17HP minimum requirement

1 HP == 0.74 KW thus 12.5KW is minimum requirement

Agni motors 95 model will output 12.5KW continuously at 56V with 5sec bursts of 22KW. The 95-R model will output 16KW continuously at 78V with 5sec bursts of ca. 30KW.

As has been stated previously the HP of an inboard engine is at the engine not at the prop whereas almost 100% of the power output of the electric motor is delivered to the prop. Furthermore if more power on a single prop is required I think 2 motors can operate in tandem with a system of pulleys and belts.

[builder dan]

I know of quite a few 28 to 32 footers around here with 10 hp diesels in them most of them are not powerful enough in any type of head sea or wind.So I cant see how a 10 hp would push a bigger boat.I would want at least 30 hp to feel happy coming into a marina full of expensive boats,or when anchor is dragging on a lee shore.

[subquanta]

Thanks everyone for your thoughts. I am going to give it a shot and see how it works out. I will pass along my experience in a few weeks.

[David_Old_Jersey]

Originally Posted by subquanta

Thanks everyone for your thoughts. I am going to give it a shot and see how it works out. I will pass along my experience in a few weeks.

Please do (it all goes into the pot of knowledge ), some pics of the installation (and boat) would also be useful.

FWIW, me is in the camp of yes!, it will be enough - just not for all of the time. How much time and how much that matters is purely down to how you choose and have to use the boat (and engine).

Am guessing that financial is the reason for the o/b choice, if my use and location allowed (i.e. not a Marina 5 miles up a river) and I really liked the boat (in design, condition and price!) me would go for the o/b as a temporary solution (even if in years!) and then either save the pennies for an inboard or for electric power (or at least have a very long hard look - from my early investigations seems that the installation can develop as budget allows).

Of course as you know, no working inboard knocks the stuffing out of the value of a boat, especially when they get older.

[SkiprJohn]

Originally Posted by cal40john

Interesting how rules of thumb change over time.

Cruising Under Sail, Eric Hiscock:

"For normal cruising purposes one horsepower per ton of displacement should be sufficient - Wanderer IV (22 tons) requires no more than 18 horsepower to give her a speed of 5 1/2 knots in quiet weather. But if the yacht is to be capable of making good progress against a headwind and sea, or to reach her maximum hull speed under power (some owners feel this is essential) at least 3 horsepower per ton will be required."


Previously posted was 1 hp per 500 lbs, which is 4 hp per 2000 lbs. Hiscock probably means long tons so about 4.5 hp per ton is the current recommendation.

Don't follow the old rule of thumb if you expect to motor at hull speed directly into 25 knot head winds.

Old rule of thumb doesn't seem to be entirely dead:
Horse Power per ton?

My boat is about 3.3 hp per ton. Works for me.

9.9 is plenty enough power. It's the height and depth in a seaway which might get you in a bit of trouble. Each wave will either put your powerhead underwater or your prop out of the water. I can't wait to see your success.

Current rated hp for inboards is way out of whack. I've seen the Perkins 4-108 rated at 50 hp or 36 hp depending on who you talk with. More honestly it should be rated at 20-25 because most folks don't rev them to 3600 rpm.

[Cheechako]

Originally Posted by SkiprJohn

9.9 is plenty enough power. It's the height and depth in a seaway which might get you in a bit of trouble. Each wave will either put your powerhead underwater or your prop out of the water. I can't wait to see your success.

Current rated hp for inboards is way out of whack. I've seen the Perkins 4-108 rated at 50 hp or 36 hp depending on who you talk with. More honestly it should be rated at 20-25 because most folks don't rev them to 3600 rpm.

Yeah, they are often rated according to usage also... or maybe I should say Derated.... Interestingly a 4-108 pushed my heavy 44 footer very well in all conditions and my max rpm was 2800. Thinking back you would think it was a little meager for that boat. The Yanmar 3QM30 in the HC 38 in my avatar didnt push the boat well at all. 27500 lb dry weight + 2500lb for excess / 30 hp (max) would be about 1000 lbs per HP. Not near enough.

[kmacdonald]

NO!!!!!!!!!!!!