Prop size question with a twist

[Bootleg23]

Good day,

I have a question about prop size and need some advice.

I have a 19 foot 1988 Starcraft 191 select with an OMC 4.3 in it. normally I wouldn't post this as there are a million posts on here with a similar configuration. However I am special as I just noticed recently that my lower unit came from a 5.7 L. The boat has been in the family for over 20 years and I recall about 15-18 years ago they replaced the lower unit as someone smoked a rock and it was destroyed.

At the time I guess they could not source a 4.3 lower unit so they mounted one off a 5.7, I also know that the 5.7 has different gear ratios then the 4.3, I am currently using a 14.5x19 stainless prop, this is the prop that has been with the boat since day 1, its in great shape.

Now on to my question, considering the different gear ratios in the lower unit, theoretically the stock recommended prop should be changed, my questions is what size?

top speed currently is 38mph @ 4000rpm, but I need to confirm the RPM as im going off memory. I know the spec is between 42-4600.

Anyway, if someone can throwing some prop sizes for me to try ill give it a go.

Thanks

Simon

 

[ThomW]

Well as a very general rule, if you are getting 4000 rpm's at WOT with normally loaded boat, you should be able to drop to a 17 pitch instead of a 19 pitch and would gain about 400 rpm's on your WOT putting you at about 4400--right where you wanna be if your motor is rated for 4200-4600.

 

[Bootleg23]

So try 14.5 x 17 then? or should I go even smaller 14.25 x 17

Thanks again

 

[ahicks]

Try it if you like!
Remember there's a TON of variables here. Things like hull design, motor heights, different prop cupping and blade designs ALL come into play here. Picking a prop is NOT an exact science! It's a HUGE set of compromises.

As far as different diameters are concerned, generally more diameter will get you better acceleration and low speed maneuvering, with less top end speed.

 

[Bootleg23]

So I am at the cottage this weekend and did some more precise testing.

using an 14.5 x 19.5 stainless prop cruising at 3k my speed was 25 mph. At WOT only hitting 3700rpm teaching top speed at 37mph (gps).

turns out I had a 15x17 prop kicking around, this one was aluminum though, I put it on right after.

3k cruising speed 22.5mph
WOT was 3800 and top speed was 35mph.

also can note the boat seems to have a better hole shot.

I was hoping for a more dramatic difference,

where can I go from here ?

thabks.

 

[QBhoy]

Hi

don’t get yourself worked up too much around diameter, unless you are going for absolute prop perfection (trust me...it can turn into a very expensive obsession). Stick with pitch for now. Diameter will make a slight difference , but only if you are aiming and measuring in 10’s of rpm and trying to totally nail down the optimum prop.

everything you have reported so far, to me makes absolute sense and reason, with regards to the ratio being more suited to an engine with more power. I’d be pretty comfortable in suggesting you try a 17” pitch 3 blade and see how she goes.

 

[Bootleg23]

Hey,

thanks for response, you mention to try a 3 blade 17 pitch prop which I have above, the 15x17 one. Are you suggesting 14.5x17 ?

 

[QBhoy]

I think you need to get some accurate numbers really. It would be very unusual for a 17” to only see 100 rpm less than a 19” with the same 3 blades.

 

[Bootleg23]

“A prop’s two dimensions each affect performance in different ways. For every inch of change in prop diameter the RPM varies by about 500 RPMs (up or down) and every inch of pitch changes the RPM by approximately 150 to 200 RPM. So if you have a prop that measures 14″x19″ and then go to one that is 15″x17″ instead, your RPMs will go down by approximately 100 RPMs (moving from 14″ to 15″ will decrease the RPMS by 500 and changing pitch from 19″ to 17″ will increase the RPMS by about 400 RPMs). The effective top speed won’t change much as these two props are about the same, but the increase of diameter will help the engine get up on plane quicker (hole shot)”

Got the above info from here https://blog.marinepartsexpress.com/2009/12/prop-sizes-and-gear-ratios/

coincidentally he uses the 2 example sizes I am dealing with, ( I made a mistake above I have a 19p not 19.5p.)

so I am experiencing what the above states while you say it’s unusual for me to experience that.

either way, I was thinking going of trying 14x17 or perhaps even a high 13 x 17?

when I say “try” I mean buy as I don’t have a shelf full of props I can simply test with, so nailing the size the first time is critical.

 

[wiigelec]

too many factors involved to turn the above generalities into doctrine