Prop selection

[pvmtsux]

I recently bought a 1998 Regal Destiny 200. It came with a 3 blade alumminum prop 14.25 with 21' pitch and a 4.3L Merricruiser I/O. I noticed that when I ran the boat the RPMs were very high, about 6000 WOT. I also noticed on my last outing that there was an unusualy grinding sound coming from the sterndrive. After further investigation I found that the prop had about 1" of play in it (up and down) the spline teeth were exposed and the nut is all the way down. My question is: 1. is this the correct prop for my boat? 2. Which direction in pich chang do I need to go to bring down the RMP's (higher or lower in inches)? 3. Why is there so much play in prop? 4. What prop (dia. and pitch) would you recomend using as I am in the market for a new one?

 

[steelespike]

Re: Prop selection

I can't tell you much about your loose prop except it is either worn out or just plain wrong.Is the prop loose or the shaft loose.Lets hope its the prop.
I'll assume the Destiny 200 is a 20 footer.If so I would say a 21 would be near the high end pitch wise.High pitch= lower rpm.Increasing pitch(higher number lowers rpm.decreasing pitch increases rpm. Of course your prop troubles could be creating the high rpm. Or perhaps the tach needs to be checked.pull the prop double check the pitch check the shaft.
I would think a 21 pitch would normally produce rpm within your recommended range.Might not work well for loads or skiing but would be close for average use.When selecting a prop diameter is a product of the design and usually not a primary consideration in the selection.If a prop is selected for your setup it will be the correct diameter.

 

[pvmtsux]

Re: Prop selection

It is most def. a loose prop not the shaft. Are there different size props depth wise (shat to end)? If so, where can I find this measurement? everything I see is diameter and pitch. Also, what will the effects of diameter change be in the boat/ engine performance?

 

[Silvertip]

Re: Prop selection

Prop selection is based on your power plant and stern drive. Props are made within a very specific diameter and pitch for that combination. In other words, a 15 inch diameter prop with 21 inches of pitch for an OMC stern drive will not fit a Mercruiser that requires the same diameter and pitch. Splines, hub diameter, hub depth and a host of other factors go into prop design. Go here to select a replacement prop:

http://boatpropellers.iboats.com/?*******=932783084&**********=592697913

Enter the information in the Stern Drive Prop boxes and you will be presented with a listing of available props for your specific drive. Now all you need to do is determine if you want (not necessarily need) stainless or aluminum, three or four blade, etc.

 

[steelespike]

Re: Prop selection

It is most def. a loose prop not the shaft. Are there different size props depth wise (shat to end)? If so, where can I find this measurement? everything I see is diameter and pitch. Also, what will the effects of diameter change be in the boat/ engine performance?

Again diameter is usually not a consideratioon in prop selection.
To keep it very basic increasing diameter will tend to lower rpm decreasing diameter will increase rpm.Occasionally diameter may be considered for other specific problems ,ventilation being one when in the extreme.Follow silvertips advice and you should be fine.Don't be afraid to ask any further questions.

 

[Skyhawk]

Re: Prop selection

Something is wrong to have that much play imo.

I have a 20' with a 4.3 Vortec with a Cobra drive and I run 14 1/4 by 21 (ss) and the WOT is right at 4800, where it should be.

6000rpm is obviously WAY too high. Sounds to me like possibly your hub is shot? If that's not it the problem would be in your prop shaft.

 

[pvmtsux]

Re: Prop selection

I took my prop off and comared to a friends and found that his has a spacer on the back where it connects to the drive. He has the same type of boat and prop that I do and mine does not have this spacer. I went to USA boats and was able to pick one up for about $15. problem 1. solved. My RPMs are running about 5400 with the new set up and I'm ok with this. Problem 2. solved. Thanks for the replies.

 

[Skyhawk]

Re: Prop selection

I took my prop off and comared to a friends and found that his has a spacer on the back where it connects to the drive. He has the same type of boat and prop that I do and mine does not have this spacer. I went to USA boats and was able to pick one up for about $15. problem 1. solved. My RPMs are running about 5400 with the new set up and I'm ok with this. Problem 2. solved. Thanks for the replies.

I think your rpm's are still too high my friend. If it were me, I'd throw a 14.25 by 23 on it and see where that hit's.

The washer your referring to is called the "thrust washer".

 

[steelespike]

Re: Prop selection

Have to agree with Skyhawk I don't think your engine will be happy at that rpm for long.I'm guessing your rated rpm is something like 4600-4800.
I think I would check the tach before a prop change.You would have to go up to a about 24" to come close to 4,800 seems like an awful lot of pitch
on a boat that size with a 4.3.