Help with prop size, please

[Sunsearcher]

Hi

I was wondering if any of you knowledgeable people could help me out on this one, I have a 1980 Glastron 266 (27 foot) twin fitted with a pair of Mercruiser 470's driving pre alpha stern drives (1.81). The boat goes very well except, at idle the boat is a pain in the marina as it the slowest it will go at idle is 8 - 10 knts, top speed is good, at 48 kts however the engines are only running at 3800 rpm WOT (speeds via gps). I have checked the props fitted and they are part number 48-36008A1 25P (I guess the 25P is the pitch). I have contacted Glastron to see what props should be fitted and they say all their records pre 1990 were lost, therefore I have to start from stage 1.

I have done a lot of research and have found that boats with similar size / weight / displacement / engines run either 15 x 21C or 15.5 x 19C. I know doing it this way is a little less than scientific, but it seems like a good place to start from

Can anyone advise the following

Is there a real difference in 15 x 21C and 15.5 x 19C
Given my performance with 25P should I go for the 21's or the 19's

Any help and advice would be appreciated

Many thanks

John

 

[junior1113]

Re: Help with prop size, please

hole shots must be suffering with those #'s 21"pitch should be perfect and should bring wot rpms up in the 4400 range where it should be. should'nt lose any top end might even gain and hole shots will be on time, and ilde speed will drop good luck

 

[Sunsearcher]

Re: Help with prop size, please

Thanks for that, 21's it will be. I will report bck in a few weeks when I have them fitted. Hole shot was a problem with the 25's, if you didn't increase throttle very very slow you just dug big cavitation hole, that sad once you were on the plane the boat performed excellent.

More later

 

[Sunsearcher]

Re: Help with prop size, please

OK, now got the boat back in the water, with 21's, wow what a change. It took a while due to finding out when I lifted the boat out that the small hydraulic manifolds fitted to the base of the transom were almost corroded away, then I found one of the hydraulic hoses to the rams was leaking and one of the rams had no threads left, so lots of bits to fix, still also changed the bellows and serviced the legs so all in all lots of bits done.

It now slips in and out of gear like butter, there is no rpm drop at idle when you put it into gear and it will potter around within the marina at 3 kts no problem. She revs freely and jumps on the pane with no cavitaion and at WOT I have 4,300 rpm on both engines and a top speed of 64kts.

Another trait has also gone, at slow speed (<10kts), when you turned there used to be lots of roll, now it just gently turns, flat with no roll. At cruise the boat feels very stable, it turns and banks and feels rock steady, before you had to constantly adjust the trims to keep the boat level. All in all a result, anyboady want to buy and old pair of 25P mercruiser props?

Thanks for all the help and suggestions, I can enjoy the water

John