Will I lose my top end speed???

[Paul Ryan]

Got my base line of 32mph (gps) at 4400rpm WOT with a 14x19...Spec WOT is 4800rpm. If I drop to a 14x17 to get up to the WOT spec, will I lose or gain my top end mph???

 

[Bondo]

Re: Will I lose my top end speed???

Ayuh,... Donno,... try it,+ let Us know...

 

[QC]

Re: Will I lose my top end speed???

Yup, Bondo is right. This could go either way and really the only way to know is data. Aluminum to Aluminum? SS to SS? Like for like?

What was your load like for the base line test? If it was light then you definitely need to consider less pitch.

 

[steelespike]

Re: Will I lose my top end speed???

If the prop has any cup you could have a GOOD prop shop remove a little of it.
I've seen some surprising results.Increased rpm and speed.

 

[642mx]

Re: Will I lose my top end speed???

Speed normally follows RPM's.

 

[45Auto]

Re: Will I lose my top end speed???

Actually, speed follows horsepower, not RPM. If you think speed follows RPM, try putting a prop with about half the pitch of your current prop on your boat. Even better, take your prop completely off. In either case your boat will turn MUCH more RPM, but don't be surprised if it's also MUCH slower!!!

I doubt if you'll see any difference. The chart below is for a Mercruiser, but the Volvo will have an identical horsepower curve. You can see the difference in power is only a couple of horsepower between 4400 and 4800 RPM (much less than 1%). So the most you will most likely see is a difference in speed of less than 1%, or in other words, less than 1/3 of a MPH.

My 16' Mark Twain tri-hullwith a Mercruiser 120 ran 33 MPH no matter what prop I tried on it. Did 33 at 3200 RPM with a 21" pitch, did 33 @ 4800 RPM with a 15" pitch. It's a function of how steep the "Power Required" curve is for your boat at that particular speed.

Borrow a prop and try it before buying one if you can.

 

[Dhadley]

Re: Will I lose my top end speed???

A lot depends on what you're running now, what you're going to be running and what motor it is. Outboards have a lot more options for set up.

Cane we assume it's going on the boat in your signature?

Can we further assume you're going with the same prop manufauturer? Same model & design?

 

[642mx]

Re: Will I lose my top end speed???

Actually, speed follows horsepower, not RPM. If you think speed follows RPM, try putting a prop with about half the pitch of your current prop on your boat. Even better, take your prop completely off. In either case your boat will turn MUCH more RPM, but don't be surprised if it's also MUCH slower!!!

I think you know what I meant. If your not revving to max RPM, your not going to run as fast as you could. Of course I don't have any charts to prove it....lmao!!

 

[QC]

Re: Will I lose my top end speed???

Most data shows the fastest combos towards the bottom of the RPM range. The point is that propeller efficiency seems to be more important than slight swings in RPM. However, propping towards the bottom is the least flexible for increased loads etc. Like many of these discussions ultimately a compromise . . .

 

[Paul Ryan]

Re: Will I lose my top end speed???

Thanks for all the input guys!!! My sig is what I'm running. My link for the project is way behind. It shows a bayliner which has been scraped. I took out the I/O, rebuilt it and installed it in another hull. On the rebuild I went back all volvo stock parts exectp the igintion which I changed to electronic. Love this little red block. Not alot of power (110hp) but I'm hoping she is going to be around for awhile worry free.
I did my base with a full tank of fuel, but it was only myself (145 soaking wet ) in the boat. She did come on plane ok but seemed to be fighting it alittle and never did make it to WOT spec of 4800. When I get 4 adults, cooler and start pulling (tube, ski, boards) I'm going to have to have the 17. I know this. Not so worried about going fast, because I know I just dont have the HP for it, but if I can get the 4800 WOT and then crusie at 4400 at or above 30mph I'd be fat.
I fully understand the compromise that I and all of us face. Sucks don't it...lol...The prop is a aluminum Michigan Wheel and with my 270 leg being the short shaft (short prop hub), I'm seeing my opions to be very very small. Been thinking about getting a used one off ebay, try it and if it works "good" then buy a new one and keep the other for a spare. I have no options for borrowing one and haven't looked into any kind of prop swap deal with iboats or anyone else.

 

[NHGuy]

Re: Will I lose my top end speed???

I'm no prop nerd but your logic is sound to me. The 17 will be your power prop for tubing and heavier loads. And you don't know til you try, but it will probably be better for the engine since it will be more easily able to get to WOT.

 

[elkhunter338]

Re: Will I lose my top end speed???

I run a 140 merc I/O in a 21 foot starcraft, heavier than normal cuddy cabin, 4 people plus kicker motor and fishing gear. I can gain topend speed with less rpms at wot, but I loose get up and go from a stop. I settled for a prop that gives me 3900 rpms wot. Most of the time I run the boat at 800-2900 rpms, rarely over 3100rpms. Higher rpms does not mean faster topend, but it will get you on a plan faster and pull heavier loads.

 

[hwsiii]

Re: Will I lose my top end speed???

Paul, I need to know the gear ratio for your out drive


H

 

[Paul Ryan]

Re: Will I lose my top end speed???

Gear ratio: 2.15...Boat weight...dont know...I found a scale the other day and am going to found out if I can get weighted out...The Bayliner I DID have was around 2100...The boat I have now has the same config but a thicker hull. So I'm thinking at my Base Test I was floating around 2500pds. But thats a guess...Am I correct in saying that since I was over proped, the prop was close to 100% efficient (slip)???

 

[QC]

Re: Will I lose my top end speed???

Am I correct in saying that since I was over proped, the prop was close to 100% efficient (slip)???

No, 90% (10% slip) would be awesome, and I show you at 13% slip which I see as very respectable . . .

 

[Paul Ryan]

Re: Will I lose my top end speed???

OooOo...10% sounds great...but then I'm not working with the right prop either...lol...OK...so let me throw this out there...The 14x19 is running at WOT of 4400. Going to a 14x17 looks like it will put me into my WOT of 4800. If we did a 15x17...In theory, have I just stayed the same as the 14x19? IE the 17 is smaller picth but the 15 dia is more water to move leaving the rpms down?

 

[hwsiii]

Re: Will I lose my top end speed???

Paul, QC is telling you the right information, prop geometry controls speed and RPM MUCH more than small diameter changes in todays props.

You might try reading this for a better understanding of changing props for better performance.

http://forums.iboats.com/showthread.php?t=365653


H

 

[Paul Ryan]

Re: Will I lose my top end speed???

Thanks H for the info. I really respect the input from everyone! The more the better! My options for prop mfg's is looking limited to either Volvo or Michigan. I have not even seen a SS prop yet. Which I guess is good. Since I know what preformace I have now with the Mich, going back with a Mich we can for the most part predicted in a postive what gains will come from it. I also see where it's a trail and error thing as well, all the differant configs and variables. Wonder if Santa has a new prop for my stocking this year

 

[hwsiii]

Re: Will I lose my top end speed???

Paul, I would recommend you getting a Solas Amita 4 blade aluminum prop in a 17" pitch for your boat. You should not lose any speed and your WOT RPM ought to be about 4,600 RPM. It will pull all the load you are talking about putting on the boat next year and your hole shot and acceleration throughout your entire RPM range will increase very much. You will NOT be able to cruise at 30 MPH, as that is a dream with the motor you have. Sorry


H