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Old 01-02-2017, 10:24   #1
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Speed.

Hi folks.
I have a 7 ton 30 foot ketch with a 40hp diesel,the engine is old and has become a bit unreliable so i have decided to have a 40hp mariner o.b. as backup. I don't use the engine a lot,but when i do it is in a very busy channel.
A new engine is not an option. Could any of our mathematical forumites tell me what approximate speed i can expect. Thanks.Ronnie.
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Old 01-02-2017, 10:35   #2
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pirate Re: Speed.

You don't need an OB that large.. a 9hp long shaft will do the job just as well and be easier to handle.. if an 8hp can push a 37ftr at 5kts you'll have no problems on just 30ft.
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Old 01-02-2017, 11:19   #3
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Speed.

For reference my iP38 at 25000 lbs is a 40 HP and still easily hits hull speed
Also a 40 HP outboard is almost certainly for a high speed boat and is geared and propped for planing speeds. You can get smaller "High Thrust" motors that are meant to push heavy things like sailboats at hull speed
Power required to push a displacement hill beyond hull speed gets crazy fast, so realistic limit is hull speed
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Old 01-02-2017, 15:07   #4
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Re: Speed.

I have an 8hp on my 29 foot 4ton boat. A 40 hp outboard is too big imo. But 8hp would be too small for you too. I know my engine is a little tired but I think boatman's assessment is a bit enthusiastic. My 8 hp will push me at 3 to 3.5 knots at about three quarter throttle. The 9.9 with the power prop and gearing would be ok, I'd guess it would get up to hull speed ok of 5kts, but if you know you need to fight some strong currents I'd consider a 20 or 25hp perhaps, with a power prop, but check mount hp ratings too of course. Btw, you won't see anything close to the fuel economy you are used to.
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Old 01-02-2017, 15:26   #5
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Re: Speed.

It is important to get a long shaft engine...Short shafts spend a lot of time pushing air in any kind of seaway.
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Old 01-02-2017, 15:36   #6
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Re: Speed.

Quote:
Originally Posted by bigman1 View Post
Hi folks.
I have a 7 ton 30 foot ketch with a 40hp diesel,the engine is old and has become a bit unreliable so i have decided to have a 40hp mariner o.b. as backup. I don't use the engine a lot,but when i do it is in a very busy channel.
A new engine is not an option. Could any of our mathematical forumites tell me what approximate speed i can expect. Thanks.Ronnie.
Would not the costs involved in acquiring the outboard, installing and bracing an outboard mount in a location not designed for it perhaps be better used to evaluate and repair the existing motor?
As others have said, you aint going to get any more speed out of it.
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Old 01-02-2017, 16:05   #7
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pirate Re: Speed.

Quote:
Originally Posted by Don C L View Post
I have an 8hp on my 29 foot 4ton boat. A 40 hp outboard is too big imo. But 8hp would be too small for you too. I know my engine is a little tired but I think boatman's assessment is a bit enthusiastic. My 8 hp will push me at 3 to 3.5 knots at about three quarter throttle. The 9.9 with the power prop and gearing would be ok, I'd guess it would get up to hull speed ok of 5kts, but if you know you need to fight some strong currents I'd consider a 20 or 25hp perhaps, with a power prop, but check mount hp ratings too of course. Btw, you won't see anything close to the fuel economy you are used to.
Well the boat that the Suzuki 8hp outboard was pushing was a Chance 37 weighing in at around 7000kgs 0r just under 16000lbs.. it powered us from Seville in Spain down to the sea and when the wind died by Lisbon it powered us the rest of the way to Fig da Foz at 5kts SOG.. also I did suggest a 9hp just used my experience with the 8hp as an example..
CHANCE 37 (WAUQUIEZ) sailboat specifications and details on sailboatdata.com
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Old 01-02-2017, 16:56   #8
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Re: Speed.

Quote:
Originally Posted by Don C L View Post
I have an 8hp on my 29 foot 4ton boat. A 40 hp outboard is too big imo. But 8hp would be too small for you too. I know my engine is a little tired but I think boatman's assessment is a bit enthusiastic. My 8 hp will push me at 3 to 3.5 knots at about three quarter throttle. The 9.9 with the power prop and gearing would be ok, I'd guess it would get up to hull speed ok of 5kts, but if you know you need to fight some strong currents I'd consider a 20 or 25hp perhaps, with a power prop, but check mount hp ratings too of course. Btw, you won't see anything close to the fuel economy you are used to.
I towed my 37 foot Rafiki 37, 12 1/2 ton, with my dink using a 2 1/2 hp yamaha and only using half throttle. No idea of the speed. I was in a channel at slack tide with little to no wind. So I admit under perfect conditions it can be done.

The boat comes with a 37 hp Volvo diesel. My friendly neighborhood surveyor commented that it was a little under powered for a lee shore or a bad seaway. When I repower he recommended a 50 hp.

Doing a quick internet search on your boat specs I was seeing around 25hp (20kw) for engine power. Best to find someone in the business that knows your boat and ask what they'd recommend.
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Old 01-02-2017, 18:44   #9
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Re: Speed.

I would sell the outboard and use the money to have the diesel rebuilt. The only problem with it seems to be reliability.
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