Prop expectations

[Truly]

Through no fault of my own, I recently damaged my prop and was unable to have it repaired. As this was an unplanned expense, I was looking for ways to save money, short of buying a new one. The dive service suggested installing a smaller, used one that the prop shop had on hand. They said it would work fine with our boat. As I trust their opinion, I said o.k. Now I am wondering what performace differences I can expect. I'll be trying it out next week. Our original prop was a 18"/13, It was replaced with a 16"/15. Any thoughts? More or less prop walk? Slower or faster start? Wide open throttle - faster or slower? Should I have trusted them?

My guess is less prop walk due to smaller size, slower start off due to less torque, faster at full throttle due to increase pitch? I am fairly new to boating so what do I know.

David
"Dolce"
Hunter 45CC w/ Yanmar 75hp

 

[Steve Dion]

David:

I think your guesses are probably good.

If you end up pulling your boat and re-proping it I would suggest you check out a Campbell prop. I doubt that the shop is giving you this prop, so you may end up with something that is NOT quite correct and still not be free.

If you end up spending some bigger bucks, then a feathering or folding prop may be a better choice.

 

[seadaddler]

Not a pro

Just for the hell of it call another prop shop and ask what they think.
Nick

 

[danjoan93]

Hi David, what reduction is your Yanmar transmission? Our 2007 45CC came with the 75 hp Yanmar equipped with a transmission reduction of 2.63 and the same prop as yours (18"x13") which i understand is okay with the 54hp. It probably should have been an 18" x 17" which I believe other manufactures install with the Yanmar 75 and similar reduction. I'm currently looking looking at a four blade VariProp 19"x15"x15".

Try this prop calculator:

http://continuouswave.com/cgi-bin/propcalc.pl

Dan

 

[petersea]

as a rule you want as large a diameter that will fit but since you went smaller but you did increase pitch my guess is you won't see a lot of difference or a least a drastic difference. There will be some but I think not enough to get upset about. I for one will be interested to hear what you think after you try it out.

 

[Truly]

thanks all for the replys, sorry to take so long getting back, just having too much fun on the boat. After going out for 4 great days in Santa Monica Bay, I can't tell too much difference. Seems like a little less prop walk. I did have a friend say to check out the max rpm, which is higher than original prop. Apparently there is some concern about over reving the engine. I'm still researching and looking into it, if anyone has an opinion about that. In the meantime I'm taking things a little slower with the engine.

David
s/v Dolce

 

[Steve Dion]

What do you consider over-reving? These engines should be run at 70-80% of the max rating. I assume that your engine is rated at 3600 rpms so something like 2500-3000 is a good range to run the engine.

I assume that this is a turbo. If you keep the engine from spinning up the turbo, you will probably conserve some fuel, but other than that these engines are VERY durable. If you start over-reving it you may also find that the temp raises a little too.

 

[petersea]

Truely, you said you have the 75 hp yanmar and with due respect to Steve the 75 hp is a jh engine and with the turbo the yanmar web site says it max rpm is at 3200 or continuous at 3100. That translates into a "cruise" operating range of 2250 to 2550 (app). If you run full wide open throttle and the rpm is around 3200 you are not under prop ed. Yanmar tachs are noted for being less then 100% accurate so I would not be concerned if your number is off by less the 100 rpm. What you need to do is get the correct engine model and spec for your engine and see where you are at with your prop. If in fact you are under prop ed a pro shop can add more pitch or go back to the original size prop. However if you do not run the engine at excessively high rpm you will be OK until you get it corrected. I think that your engine has a rev limiter so that you can not over-speed the engine to the point where it will fly apart but you may want to get with a good yanmar mechanic to make sure of this. It actually sound like you are pretty close to where you want to be but need to know your numbers before you can be certain.

http://www.yanmarmarine.com/uploads/products/pdf/JH/4JH4-TE.pdf

 

[Steve Dion]

Peter:

I suppose it depends on which motor he has. Regardless the correct RPM is around 70-80% of max rated rpm. David will be able to determine if he is seeing the same speeds with the smaller prop. I would suggest that he should have the correct prop for the engine. When you are trying to motor through rough seas is probably when he will notice it and it will not be there.

http://www.marinedieseldirect.com/repower/specs/yanmar/4JH3_TE.pdf

http://shop.torresen.com/marine_die...pagetitle=Yanmar+Marine+Engine+Identification++