prop mystery

[igofish]

2008 lund classic 1660 side console
16' 6"
900+ lbs dry weight
60hp mercury 4 stroke efi (not bigfoot)
WOT range 5500 to 6000 rpm
the mercury aluminum 3 blade prop from the dealer was 10 3/8 inch diameter 13" pitch
with dealer prop WOT was 6100 @ 36 mph

The problem was getting on plane with more than 2 people in hte boat. The rear end would squat, and it would take forever to plane unless we moved someone to the front of the boat.
I decided to try a 4 blade prop to get better hole shot and stay on plane better at lower speed. I read that you should drop 1 inch in pitch when moving to a 4 blade, but snce I was a bit over the max rpm, I decided to try the Solas 4 blade aluminum at the same pitch. The new prop is 10 1/2" inces in diameter, and the pitch is still 13.
I was expecting a 200 to 300 rpm drop since I didn't drop down in pitch, however with the new prop WOT is only 5400 rpm but I only lost 2 mph to 34 mph. WOT rpm is not even in the recommended range now. What did I do wrong? Is one of the props not at the specified pitch? It seems odd to lose 700 rpm and only drop 2 mph. Can I measure the pitch somehow to see if I got the right prop?
Will the 12" pitch 10 4/5" diameter Solas 4 blade aluminum prop put me in the correct range? Are there better alternatives?
Are there alternatives to buying another new prop? For example could the new 4 blade prop be adjusted at a prop shop to a lower pitch, or would I be better off buying another new one? Any idea what prop shops charge for that type of work?
Other suggestions?
thanks in advance

 

[steelespike]

Re: prop mystery

Any time you change Brands and design you add more unknowns to prop results.
Typically if any most reccomend a 1" drop in pitch when going to a 4 blade.
It is a drastic change; perhaps the Merc prop was doctored or has no cup and the Solas has more cup.A prop shop can measure both props.And give you an idea of details in the design.They should be able to modify the prop if it seems cost effective.Unless the boat is really stern heavy the max rpm and poor hole shot don't really go together usually.Do you trim in for hole shot?
The rule of thumb of about 200rpm per inch is just that a rough estimate.
I would say that a 1" pitch change won't get you even near what you need.
But something doesn't add up. Have you checked the new prop marking usually on the barrel or under the prop nut.

 

[Viking...]

Re: prop mystery

You changed several variables there, when you changed props.
Some of the changes you are obviously aware of, some maybe less so.

First of all, changing from one manufacturer to another; expect different performance characteristics.
Changing from 3 blade to 4, you are already expecting different parameters, and have noted them.
Diameter change is small, but will compound other variables.

At least now you have established a couple of baselines, one for 3 blade manufacturer props, and one for 4 blade Solas props.
Just to confuse things more, the material itself will change the performance characteristics.

Most of this you may have already known.
I think you were just surprised by how much difference seemingly small changes can make, in your case it was an 11.5% change in the rpm at WOT.

Since you are very close to your indicated range for WOT, any changes you make now should be as small as possible.

Good luck mate!

 

[igofish]

Re: prop mystery

Thanks for the responses. Yes I trim in all the way for hole shot. The boat does seem a bit stern heavy, as moving someone up front helps. I read somewhere about the solas 4 blade helping with stern lift which is another reason I wanted to try it. Yes I checked the stampings on the props and both say 13" pitch. Soooooo, if I wanted to try another Solas prop, any opinions on whether I should go to a 12 pitch, or all the way down to 11?

Thanks again.

 

[steelespike]

Re: prop mystery

An 11 would really get you some holeshot and rpm But I think might knock even more off the top end.If the 4bld. helped your hole shot the 12 would be the conservative way to go and get even more holeshot and rpm without losing too much speed.

 

[hwsiii]

Re: prop mystery

Igo, Spike and Viking both make extremely good points and viable alternatives. But if you will take both props and get me some GPS speed and RPM numbers from 3,000 RPM up to WOT on both props at 500 RPM intervals, I may be able to give a better explanation of what is happening in particular.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057




H

 

[igofish]

Re: prop mystery

Thanks for all the suggestions. It may be a while before I get a chance to get the speed measurements. I can only get out on the weekends. Am I correct in thinking that going from 13 down to 11" pitch in the same SOLAS 4 blade would get me back closer to the 6000 RPM max WOT? (it is at 5400 with the 13). The 12" would only get me to 5600 or 5700 right?

From what I read it seems as though getting it close to 6000 would be best for the life of the engine also. I was just suprised at how much the RPM dropped. I think I read that post before, and was expecting a 200 to 300 rpm drop and was suprised when it dropped 600 to 700 instead. Otherwise I liked the prop, it seemed much smoother than the old one, and handled better at speed. I don't mind losing some top end speed to get up on plane faster.

Do I need to wory about the 11" prop running too fast/over revving the engine? I also noticed the diameters seem to increase slightly with the pitch drop in the solas 4 blade props, will that affect the outcome also?

Thanks again.

 

[reelnative]

Re: prop mystery

you can take it to a prop shop and have them take it 2 degrees eather way up or down thats about the max a shop can do to it, if it were me I would go and have them take it down to 12 and try it if you need more then have them take it down to 11, also is there much cup in the prop ? a lot of cupping will lower your rpms you might want to have the prop shop look at the prop and tell you just what they can do for you