Prop selection

[sparkroost]

Could someone give me a basic range of a prop pitch in a 3 blade SS. I am guessing I have 120hp at the prop(73' 135 V4 rude(crossflow), it has been ported with square ports, forged pistons, stuffed crankcase, B.B. exhaust) it's on a 74 wellcraft 17" center console V-hull. I have 6" setback with a powerlift jack plate. At the moment the best X-dimention is cav.plate 3" above the hull, this is with a 4 blade ss prop. I am getting 36mph with this prop. The 4 blade prop turns an easy 6200 rpm, if not more, but I don't know the pitch. I have also tried a 3 blade aluminum. Pitch unknown. 41mph @5900. I can go 6200 with this prop also. There is a number on the hub, would this tell me pitch or is that just a number for the hub?

 

[SeaDawg]

Re: Prop selection

Do a search on "pitch", and it should come back with some recommendations from earlier posts by myself and others.<br /><br />I don't make it out here to check the site like I used to in the past, too busy at work, sorry!<br /><br />You need to figure out your final drive ratio (how many RPMs your prop turns compared to your engine RPMs) in order to semi-accurately calculate your prop pitch. Then, there are several web sites that will let you plug in your RPM, your final drive ratio, your MPH, and prop slip (figure 10% for starters), and they will then calculate your prop pitch.<br /><br />Ron

 

[Spidybot]

Re: Prop selection

Sparkroost,<br /><br />Your setup is beyond 'rule of thumb' -theories.<br /><br />Important here is to find the prop that lands you near the high end of rpm @ wot range (stated as 5000-6000 for your motor), at the trim and load you normally use. Your gear ratio is 2.25.<br /><br />Your prop should have numbers stamped into the hub, between blades or at the front end. These should make ID possible.<br /><br />If you like check www.dorthe.homepage.dk for a free copy of Quicksilvers booklet on props. The page even has a prop calaculator.

 

[sparkroost]

Re: Prop selection

UU, Thanks for the info. Are you positive on the gear ratio? If mine is 2.25:1 then this is better for larger pitch.(I can check it this weekend to double check)<br /><br />I have looked over the 3 props I have and none of them have any stampings that I can see except for the number on the hub.<br /><br />Even if I did have the pitch for the prop I have, I would not be able to make an accurate decision based on RPM because I have not wanted to go past 6200 RPM.(not sure really how many more rpm's I can get, just know it's more than 6200 now.)<br /><br />What discourages me is that when you look at prop manufacturers for a 135hp motor they give a max pitch prop available up to 21" pitch. The 22" and above are for the 150+hp 15 spline motors.(not that they don't make them, but the popularity of higher than 21 is not there)<br /><br />@23" pitch, 10% slip, 5900 rpm is giving 52+mph. that is plenty fast.<br /><br />I guess what I was looking for was someone who has tried these combinations already and could give advice on how that prop seemed to work or others in that same prop range.<br /><br />I just bought a 389514 23" 12 3/4 SST-2 prop. If it doesn't work, I will have to lose some weight, lol...I may have to re-pitch it, if it's worth it or just sell it. It also makes it difficult in choosing a prop due to different cupping, raking, blade tip design, etc.<br /><br />I don't see alot of people using the multi-pitch props or talking about them(the mercury ones, not the composite flex ones). sounds neat to have a 13 pitch prop for takeoff and move it's way up to 23!

 

[sparkroost]

Re: Prop selection

Sorry seadawg, left a Thank You out for you.. so.. here it is..<br /><br />THANK YOU TOO

 

[SeaDawg]

Re: Prop selection

No thanks needed, wish I could help more. Actually, you may want to call around to the local prop repair shops, because in my area, they will let you try different ones, as long as you agree to buy one in the end.<br /><br />I calculated what I thought I needed, tried it first, and then went back and tried a higher and lower pitch for grins.<br /><br />Turned out, I nicked the correct one for my boat first (the one that would just redline the engine with the lightest possible boat load), but it felt good to see it with the other props too.<br /><br />Ron

 

[Spidybot]

Re: Prop selection

I got the gear ratio from a Johnson, but that should cover your Evinrude, too.<br /><br />It isn't easy to take others experiences over to a boat like yours as you're beyond standard setup in both motor specs and jackplate.<br /><br />For your info I can tell, that I have a Merc 115 on a 16' Sea Ray. The transom is notched and the ex-dimension is about 1" high. It came standard woth a 23" alu 3-blade which ventilated heavily. No trim or cornering possible. Couldn't read max rpm as it would overrev due to ventilation. Giving full throttle from zero woyldn't hardly move the boat but make a whole sea of whiped white water behind it.<br />Switched to a 22" HighFive and got the smoothest accelleration, all the trim and cornering I could dream of and a great top end. Could have better hole shot but I don't want to sacrifice the cruising pleasure. My power/weight ratio is so, that lugging is not an issue.<br /><br />The switching props have been discussed (Bondo has one and is happy), but most opinions are agains due to the price. Also, they are available for big gearhouses only (think the shifting mechanism won't fit in the small hub).<br /><br />There are props of all kinds for your motor as well. May not be off shelf stock and for sure costs a lot.

 

[sparkroost]

Re: Prop selection

Appreciate the info. Can't wait to get the new/old prop in the mail. I am already thinking that it won't work. Not a bad price though. I will have a starting point at least. I will let ya know how the old girl likes it.