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Old 08-11-2010, 09:10   #1
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Opinions on Repower (hp)

Looking for opinions on a boat that we looked at last week.

The vessel in question is a 38' (41.5' LOA), 21,500#, full-keel cutter about 21 years old. She came from from the factory with a Yanmar 4JHBE (44hp) and has recently been repowered with a brand new Yanmar 3JHE (38hp per Yanmar although the listing broker says it's 40). The same factory uses the 3JHE in a similar model, but it is 3 feet shorter and more than 3,000# lighter.

My concern is that having the 3JHE she might be a little underpowered now. Your thoughts?
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Old 08-11-2010, 09:54   #2
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It sounds like a 10% loss of power, and I don't think that's at the prop. I'd be concerned and probably want to talk to a propeller dealer to make sure the current prop is adequate for the new engine.

I'd think you'd have to run the engine at a bit higher RPM to get similar speeds to the 4JHBE. I'd also think you might experience a bit slower speed in adverse conditions.
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Old 08-11-2010, 09:55   #3
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Having voyaged more than 10,000 miles in a Westsail 32 with an MD2B Volvo that claimed something like 25hp, 38 hp would seem overpowered to me. I delivered two Westsails with, one with an MD3B and the other with a Perkins 4-107, you should be just fine.

FWIW, the idiot owner of the Perkins equipped boat was able to stop the boat on a reach with full working sails up in more than 20 knots of wind. He sailed into Newport Beach in a Santa Ana wind, single handed, and went by us at hull speed under sail. Unfortunately, he came to the end of the harbor and had to stop. He just put it in reverse, redlined it, and then dropped the sails.
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Old 08-11-2010, 19:20   #4
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you could always call yanmar, mack boring is their east coast distributor located in union new jersey. their sales department is very helpful and can tell you exact differences and hp considerations between those 2 engines. i have the yan3JH4E in a 15,000# boat and it is more than adequate. since you're looking at an older boat, imho it's more important to find one with a strong engine with low hours. and remeber , in the 'olden days', an 11 hp diesel was thought to be adequate to power an 11,000 # boat...
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Old 08-11-2010, 20:32   #5
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When you consider your repower, don't forget to consider what the currents are in your favorite/expected cruising areas. I would want a much larger engine in the PNW than I would in NC, for example.
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Old 08-11-2010, 22:21   #6
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Quote:
Originally Posted by DWJensen View Post
Looking for opinions on a boat that we looked at last week.

The vessel in question is a 38' (41.5' LOA), 21,500#, full-keel cutter about 21 years old. She came from from the factory with a Yanmar 4JHBE (44hp) and has recently been repowered with a brand new Yanmar 3JHE (38hp per Yanmar although the listing broker says it's 40). The same factory uses the 3JHE in a similar model, but it is 3 feet shorter and more than 3,000# lighter.

My concern is that having the 3JHE she might be a little underpowered now. Your thoughts?
Hello Daz,
What brand is your boat ? Different brands with the same length and displacement may require different HP and Prop sizes
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Old 09-11-2010, 08:05   #7
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It's not a terrible reduction in power. Most boaters probably waste that much HP just by using the wrong propeller. Be careful to get the right propeller for your boat and engine and I think you'll be fine.
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Old 09-11-2010, 12:10   #8
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Like denverdon above.
At the most, you might have to reprop and get another 1'' of pitch
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Old 09-11-2010, 12:16   #9
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1.5 x the displacement is the usual calculation more or less. so 38 is pretty accurate.
2x the displacement to be on the safe side
If you have extra heavy producing alternator(s), you need extra HP so play up.

propeller matching the gear ratio is critical to engine performance and wear.

and remember, your goal is to get up to max hull speed, and all the extra power or RPMs above that is wasted energy and strain on the engine and the mechanicals.

Bottom line, as far as I can tell with the given info, is you are fine.
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Old 09-11-2010, 16:10   #10
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The 38ft is that length on deck or LWL ?
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Old 09-11-2010, 16:21   #11
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Thanks for the comments and opinions everyone.

As I said in my initial post, the boat is not mine, so I'd rather not say on the forum what the make is. Here are the specs though:

LOA: 41'6"
LOD: 38'
LWL: 33'
Beam 12'8"
Displacement: 21,500#
Ballast: 10,000#
Draft: 5'
Air draft 51'

There has been no mention of anything being done to the prop (in the listing, in any of the owner's ads, or when we looked at the boat in person), so I assume it is the original.

She is one of eight of this model that we've looked at in person and is in the top three on our list of contenders. I just wasn't sure about the new engine size. Based on everyone's comments, we'll keep her on the list for now and will do some more research.

Thanks again,
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Old 09-11-2010, 19:06   #12
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Daz, thanks for the LWL of 33ft - this measurement is probably one of the most important when trying to evaluate engine and transmission requirements. Only from the LWL can one calculate hull speed -
The formula :- Square root of LWL x 1.34 = Hull Speed (SOG)
Square Root of 33 = 5.7446 x 1,34 = 7.7 knots
Here's a link FROUDE NUMBER SPEED LENGTH RATIO AND HULL FORM WAVE DRAG RESISTANCE

The other most important measurement is what torque does the engine produce at a specified RPM. Torque in ft/lbs can be arrived at by dividing HP by RPM and multiplying the answer by a factor of 5250
Therefore the Yanmar 3JHE output 39HP at 3,000 RPM produces 68.25 ft/lbs. Whereas the older Yanmar 4JHBE output 44HP at 3600 RPM produces 64.2 ft/lbs of torque.
The above reflects the history of diesel engines where the manufactures have been able to produce lighter smaller engines and yet producing better performance.
So if that boat with its 3JHE naturally aspirated engine can achieve its Hull speed of 7.7 knots (assuming it has the correct size and pitched propellor) and because it's new - then if all the other bits and pieces are in similar shape, worth doing a sea trial.
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Old 09-11-2010, 20:15   #13
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HP = ((((displacementInLBS)/2240)*75)*(1.4*(((LWL)^0.5)))*0.003)/0.45

if you want a technical formula
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Old 09-11-2010, 20:47   #14
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Hi Salty,
From the specs given what HP does the formula produce ?
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