Author Topic: New prop  (Read 4824 times)

Valor

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New prop
« on: November 02, 2017, 07:12:36 PM »
Hi Everyone,
Hoping someone has some information on a new prop for Valor.  I have a Yanmar 3gm30f diesel spinning a 15 x 8 prop. Seems like my speed under motor is off a bit. I have the ability to bring the engine to max revs which is 3400 rpm, so it not bogging down or low on power and i normally run the engine good when  i cruise between 2400 and 2800 rpm. The engine is in perfect running order. Is there anyone out there running a similar sized (27-30hp) diesel that can tell me what prop theyre using?

I was on a friends Ericson 38 with 6 people on it and it was easily doing 6-7 knots recently. I should be able to steam stronger then 5 knots. Opinions please!

Thanks
Michael
Michael M

1978 Pearson 323 Hull #108.
Yanmar 3GM30F

selene

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Re: New prop
« Reply #1 on: November 02, 2017, 08:27:30 PM »
Hi Michael,

I have a Westerbeke 30B Three, 27 hp, so similar.

Originally 13 x 9 12 3-bladed prop.

I upgraded to a 15 x 9 feathering max-prop; a great improvement under power and especially under sail. On a calm day, flat water, clean bottom:

Old prop:
2400rpm         4.4kn
2600                 4.8
2800                 5.0

New Prop:
2400rpm         5.9kn
2600                 6.4
2800                 6.6

Wow. And the ol' Iron Genny has no problems spinning this larger prop.

As you know, an Ericson 38 has a lot more waterline than us, so I would expect them to steam ~20% faster. And a dirty bottom will slow you down. And as for the people on board...well, out boat weights 13,000 lbs dry weight, so maybe 14k lbs. Ericsons are proportionally lighter than the 323; a 38 weights around only 15,000lbs dry weight.  So with either boat, the weight of average-sized people is a negligible weight increase, and isn't going to effect the speed under power that much (if at all). But that extra 6 feet in the waterline...for ~1000 lbs weight...that is another story.

I race against an Ericson 30 (amongst others)...can't get close, most of the time.  Still win sometimes - give me a broad reach and a bit of chop :-)


Valor

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Re: New prop
« Reply #2 on: November 02, 2017, 09:37:19 PM »
Hey there Selene,

Thanks for that bit of info. Quick few questions.
Is 2800 rpm your max rpm or is that the high side cruise rpm?
Did you lose the ability to touch max rpm with a 15x9?
If you remove the feathering aspect from the equasion,  is it the same 15x9 as a fix 3 blade 15x9, or does it have or do something magical to the blades or pitch ect?

As always, i apreciate the advice.

Michael
Michael M

1978 Pearson 323 Hull #108.
Yanmar 3GM30F

selene

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Re: New prop
« Reply #3 on: November 03, 2017, 09:52:08 PM »
Okay, so...

I didn't put in the entire table - I did crank her up to 3000 rpm, getting 6.8kn with the new prop. 

I normally cruise around 2400-2600rpm, and saw no need to go higher. This is because optimal engine speed is generally held to be 60-80% of rated rpm. Which for my engine means 2400-2800rpm.  Above that, you are burning a lot of fuel for limited extra speed.

As I recall from my testing, with higher rpm the speed increase started flattening out as I pushed over 3000 rpm - which is entirely reasonable as the max hullspeed of a P323 is around 7kn. So pushing the engine harder to get 0.2kn did not seem worth it!

Taking the feathering out of the equations is difficult, as I did increase the prop size from 13" to 15", which will have accounted for a lot of the speed improvements.  Not clear to me if the pitch effected things much - I seem to get a little more cavitation with the new prop until the boat reaches the right speed.

Back to your original question - the Ericson 38 is 30.4 feet at the waterline, which puts its hull speed at 7.4kn....