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Old 01-05-2016, 01:23   #1
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Engine Size Considerstion

I've been given a price today on a Kubutto engine. My current engine is a vp2003T, which is 38hp. I've given given a price for a 35 or a 50hp. The difference between the two is only $1000. Tentatively comparing the engine bay sizes today I think the 50 will fit.

But, woukd I give myself any advantage in going to 50hp, or will it be too much?

Also, would the going down to a 35hp be a negative step from the current 38hp (VP2003T)
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Old 01-05-2016, 01:59   #2
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Re: Engine Size Considerstion

The 35hp is probably close enough that only fighting the worst conditions would you be able to tell the difference and even then it's only 10% less so probably not much of a difference.


One option to consider is dropping down an inch or two on the prop pitch. We switched from a 40hp to 25hp but also dropped from an 11" to an 8" pitch. We still get to hull speed just fine but actually feel like we have more thrust in docking situations. (This is outboard power so not exactly the same and the 40hp was probably overkill as hull speed wasn't even 1/2 throttle).


The only reason I might consider the 50hp is if you plan on a lot of heavy duty accessories (couple big alternators, engine driven compressors, etc...) Even then most of those can be decoupled for motoring.
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Old 01-05-2016, 02:08   #3
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Re: Engine Size Considerstion

I went for a 30hp for my 40 footer. I am happy enough, Ideally a 35 or 38 would be ideal for me. The 30 cant quite push her to hull speed, without working hard, but she cruises along at 6 or 7 knots comfortably and economically. And can punch into it well enough for me.

In your case its a tough call given the minimal extra dollars. The 35 is probably the right engine for her, but the extra hp could be useful if you like to motor into strong winds rather than motorsail.

The downsides of the 50hp are more weight, less room and a need to run it hard every so often, burning more fuel to keep it happy. Have you got room for the bigger prop? Is the shaft big enough? Exhaust and water intake may need to be upsized by one for the bigger motor.
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Old 01-05-2016, 02:13   #4
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Re: Engine Size Considerstion

I'm under the impression that when correctly propped, you might get similar speed out of either engine, but when you are bashing into chop and wind, the more powerful engine will allow you to accelerate better and therefore maintain a better speed.
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Old 01-05-2016, 02:18   #5
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Re: Engine Size Considerstion

I suspect it has been many years since your VP actually put out 38hp.
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Old 01-05-2016, 02:46   #6
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Re: Engine Size Considerstion

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Originally Posted by Snowpetrel View Post
I went for a 30hp for my 40 footer. I am happy enough, Ideally a 35 or 38 would be ideal for me. The 30 cant quite push her to hull speed, without working hard, but she cruises along at 6 or 7 knots comfortably and economically. And can punch into it well enough for me.

In your case its a tough call given the minimal extra dollars. The 35 is probably the right engine for her, but the extra hp could be useful if you like to motor into strong winds rather than motorsail.

The downsides of the 50hp are more weight, less room and a need to run it hard every so often, burning more fuel to keep it happy. Have you got room for the bigger prop? Is the shaft big enough? Exhaust and water intake may need to be upsized by one for the bigger motor.
I hadn't anticipated needing a bigger prop or a new shaft? I'd rather avoid the extra cost.

My Volvo vp has a 3 inch water system, why I don't know. But with the 35,38 or 50, it's only a 2 inch system.

The price was from VW that you recommended. He only does 35's and 50's, not the 38 which is go for if I go for the red Beta.

I'll compare weights.
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Old 01-05-2016, 02:48   #7
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Re: Engine Size Considerstion

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I suspect it has been many years since your VP actually put out 38hp.
That's probably true
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Old 01-05-2016, 02:51   #8
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Re: Engine Size Considerstion

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I'm under the impression that when correctly propped, you might get similar speed out of either engine, but when you are bashing into chop and wind, the more powerful engine will allow you to accelerate better and therefore maintain a better speed.
hmm, might be worth it then. Both engines are similar width and height, but I'd have to modify the front of my engine bay by 100mm to take the 50. Not really hard to do as I just have to move my battery box forward and widen the first step in.
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Old 01-05-2016, 02:56   #9
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Re: Engine Size Considerstion

Weights

35 is 155kg

38 is 168kg

50 is 246kg

Quite a bit heavier is the 50.

I've also just noticed that Beta give a five year warranty
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Old 01-05-2016, 03:10   #10
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Re: Engine Size Considerstion

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I hadn't anticipated needing a bigger prop or a new shaft? I'd rather avoid the extra cost.
Even if the new motor is the same power as the old, there could be a different gear box ratio with the new and that might require a different prop anyway.
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Old 01-05-2016, 03:16   #11
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Re: Engine Size Considerstion

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Even if the new motor is the same power as the old, there could be a different gear box ratio with the new and that might require a different prop anyway.
Ok. The choice of gear boxes come in different ratio's, but I don't know what they mean. I'll give the Beta supplier and the WM supplier tomorrow and get all the options.
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Old 01-05-2016, 03:30   #12
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Re: Engine Size Considerstion

Check the prop Rotation direction as well, though some gearboxes let you run them in reverse they normally are derated slightly to cope with the extra heat generated.

The gearbox size, type and cooling is a big deal.

Many yacht engines come with undersize gearboxes and they cook and fail pretty quickly.
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Old 01-05-2016, 03:34   #13
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Re: Engine Size Considerstion

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hmm, might be worth it then. Both engines are similar width and height, but I'd have to modify the front of my engine bay by 100mm to take the 50. Not really hard to do as I just have to move my battery box forward and widen the first step in.
The issue isn't so much acceleration as the engine bogging down as you punch thru a wave and then it can't recover before the next wave. A bigger engine can muscle thru that without bogging down.

Alternatively a lower pitch prop or a deeper gear ratio can provide a similar effect. It's like putting a 40hp tractor in a towing contest vs a 400hp Ferrari. The tractor will win but it will never go as fast as the ferrai on the open road. The only thing you need to do is check what the cruising RPM will be with the new pitch/gear ratio. If hull speed will require 3800rpm and the recommended max for the motor is 3600rpm, that's a problem. As long as cruising speed results in rpm near the peak engine efficiency (not peak HP) and the engine can hit full RPM at max throttle, you should be good.
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Old 01-05-2016, 03:36   #14
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Re: Engine Size Considerstion

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Originally Posted by Snowpetrel View Post
Check the prop Rotation direction as well, though some gearboxes let you run them in reverse they normally are derated slightly to cope with the extra heat generated.

The gearbox size, type and cooling is a big deal.

Many yacht engines come with undersize gearboxes and they cook and fail pretty quickly.
Ok, will do, and to avoid Hurth gearboxes.
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Old 01-05-2016, 04:02   #15
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Re: Engine Size Considerstion

RC;

I would suggest you also need talk to a prop supplier. First you need to know whether your existing prop will suit if you go with the new 35hp engine. This will depend to some extent on the available gearbox ratios compared with what you have now.

Second you need to know what prop is required if you go up to 50hp: it will certainly need to be changed from your existing one. And that brings into play things like a possible increase in shaft diameter, possible upsizing of the coupling, and whether there is enough clearance down below to swing a (probably) larger diameter prop.

Prop suppliers will do the sizing calcs for you. I had good support from Austral in Sydney for repropping of my Compass 28.

Cheers, Graeme
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