Change prop after hydrofoil?

[tchpet2]

Hi all,

So, second tme in water after installing hydrofoil and I hit a stump. Needless to say, I needed a replacement hydrofoil and a new prop

However, I have a dilemma. After putting on the hydrofoil, my RPM went way down at WOT. I was running 5200-5400 and now it is 3500 - 4000. So, should I change prop, or is it normal for rpm to go down with hydrofoil? I am guessing this is normal, but not sure if I need to get the rpms back up.

When I gun it, from a standstill, rpm will go as high as 4700, but not to 5300.....I think it is because boat planes pretty quickly, which is a good thing

Boat is a 17ft lowe bass boat with a 25hp on it. Current prop is 3 blade, 10.5 dia. 11 pitch. I am hitting 21mph with current prop at WOT.

SO should I change to different pitch or order same one as I have?

Thanks.

 

[ondarvr]

Re: Change prop after hydrofoil?

What were you trying to fix with the foil, or was there an issue to start with?

 

[tchpet2]

Re: Change prop after hydrofoil?

Just wanted to get on plane quickly. Place I fish has lots of shallow places.........thus the stump, (although I have been pretty good about staying away from them) but I really wanted to get the boat up quick out of the junk.

 

[hwsiii]

Re: Change prop after hydrofoil?

Pet, something doesn't math up with the statements you made above, if you would like my help please fill out this form.

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


If you would prefer to not go to this much trouble there are many other people who will come along and gladly help you.

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
2a.What is the maximum recommended HP for your boat
3. Number of people and gallons of gas normally on boat and what do you use the boat for
4. Year, make. manufacturer and model of motor and recommended Wide Open Throttle (WOT) range
5. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
6. Anti-ventilation Plate height above keel of boat if it is an outboard in inches, use a straight edge under the keel and sticking out to the anti ventilation plate for a reference
Motor Transom Height

7. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating.
RPM _________ Speed ______ GPS ______ No. of people ______ Gal. Gas ________
8. Make, model, diameter, pitch, number of blades and whether SS or aluminum prop and does it have a hydrafoil, dolefin or trim tabs
9. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way by just pushing the throttle at the helm and not at the carburetor, checked compression, and looked at the plugs and checked spark as well as timing and advance, all of the foregoing could be the reason your prop is not attaining full RPM
10. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations and does the prop show any damage that you can see

REMEMBER, The numbers I give you will be NO better than the information you give me. I use a computer program I designed to find the correct pitch and then knowledge to pick a Better prop design for your boat.

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.



H

 

[tchpet2]

Re: Change prop after hydrofoil?

I will do my best.......but boat is stored somewhere else. By the way, I have already changed the prop (from original one) to get rpms up to 5300 before I added the foil.

1. 1995, Lowe, 17ft bass boat
2. Only thing I know is it is 17ft, alum. with alum. decks.
3. 2 people, 10 gallons of fuel in tank, used for fishing........oh, and drinking better at the drinking than the fishing
4. 1995 Johnson 25hp......5400 (if I remember right)
5. 25hp and I don't know
6. even
7. rpm around 3700 but it varies, 21 mph by GPS, 2 people, 10 gallons of gas
8. Solas, Amita 3, OB3X10.5X11RB, alumin
9. motor is all good to go
10. since I added foil, lost rpm at wot. boat is as fast as ever, but rpm is much lower. might be a good thing, but from what i have been reading, should try to keep it as close to specs at wot........right?

 

[hwsiii]

Re: Change prop after hydrofoil?

Pet, you need to borrow a mechanics tach to check your tachometer, because you definitely are not turning 3,700 RPM and doing 21 MPH if you gear ratio is 2.15:1 as I show, please verify this.

Try turning the switch on the back of the tach, they will build up corrosion over time and turning the dial helps clean it. I show you with a -17% prop slip, which is impossible.

Prop Slip

H

 

[steelespike]

Re: Change prop after hydrofoil?

I wonder if you motor is low and your dragging the whole foil threough the water.
Also could be too much lift causing the boat to bury the bow.
Something like smart tabs are a better solution.The foil can be very effective but it also can cause problems.If you hit something with the foil you could break a mount
or the antivent(cavitation) plate. I would change weight distribution and maybe trim in a notch,Consider a 4 blade prop.Raising the motor may improve hole shot and top end.

 

[tchpet2]

Re: Change prop after hydrofoil?

I am using a different tack........one on boat wasn't accurate at all. I am using one that hooks to plug wire and then ground. Works really good.

I can't figure out why the wot would go down after a foil. Even if the foil is going through the water at plane, wouldn't the rpm go up as the motor would be pushing harder? It is as if the rpms go up, boat on plane, rpms go down.

 

[ondarvr]

Re: Change prop after hydrofoil?

Working harder doesn't equate to higher RPMs.

You need to take a couple steps back and understand more of what you to do and what you need to do.

The foil will not work well by just bolting it on, you will need to adjust motor height for good performance, that is "if" it will even help at all. Its very common to lose speed and RPM when you don't adjust motor height.

Like others said, you need to be certain of your speed, RPM and the prop before you start making changes and then adjust one thing at a time and see if you get the results you want.

 

[tchpet2]

Re: Change prop after hydrofoil?

ok............here are ALL the steps I took....

Changed prop to get 5300 RPM (range of specs) Ran it for 1/2 season (last summer) like this.........speed was 21mph, but took long time to get on plane.

Ran boat this season (out once) @ 21 mph, 5300 RPM

Added foil.........speed went down to 19 mph, 3700-3800 RPM

Dropped motor 1"..........speed went to 21mph, RPM stayed at 3700-3800 RPM

Thus.....I have already adjusted motor. I don't remember pitch of old prop, but I used calculator to get new prop (before foil) and it brought RPM up to where it should be. This was before foil.

I know lots of guys don't like the foils. However, even if I can't get RPMs up, I will still use it because it does as advertised............puts the boat on plane very quickly. Much quicker than I could get the boat on plane with just the prop change. And yes, I have trimmed the motor all over the place It is trimmed at the "best" spot I have been able to find for top end speed and getting on plane.

 

[Gary H NC]

Re: Change prop after hydrofoil?

I agree on the point you need to raise the motor up,not drop it.
The foil needs to be just out of the water or most of it while on plane.

 

[ondarvr]

Re: Change prop after hydrofoil?

Dropped or raised the motor....dropping is the wrong direction.

If the prop was ventilating a great deal before adding the foil it is possible that the foil stopped it, this will cause the RPMs to drop. This is actually the correct use for the foil, but it normally affects low speed ventilation not high speed. Other than that there really isn't a way to drop 1,500 RPM and keep the exact same speed.

 

[tchpet2]

Re: Change prop after hydrofoil?

Oops........typo I meant raised the motor 1", not dropped it. Basically I went as high as I could (the top clamps are right at the edge of the transom). Raising it took the foil out of the water at wot which gave me back the 3mph.

 

[ondarvr]

Re: Change prop after hydrofoil?

If the foil is out of the water at speed, then it will no effect on your RPMs due to drag, this sort of leaves the drop in RPM to something else, possibly a mistake or incorrect reading. By raising the motor your speed and RPM's will normally go up.

As I said before, dropping 1,500 rpm and having the exact same speed with the same prop would be difficult to do in a situation like this.

 

[tchpet2]

Re: Change prop after hydrofoil?

Well, I figured out my RPM problem. Changed the spark plugs and it is now back to 5500 RPM. Must have been something with the plugs....they didn't look bad at all, but must have been something wrong with them.

I did take off the hydrofoil though and went with the smart tabs. I had the boat out for the first time since putting on the tabs and had them set too stiff. So I am looking forward to next time now that I adjusted them to a lower settig.

Of course, I also hit a stump and dinged up the prop. I am thinking about going to a 4 blade prop, but not sure as I currently have a pitch of 11 and the only 4 blade for my motor that I can find is at a pitch of 12. I am ok with my RPM going down to 5300-5400, but don't want to drop more than that. So, should I stick with 3 blade or try the 4 blade?

 

[45Auto]

Re: Change prop after hydrofoil?

speed went to 21mph, RPM stayed at 3700-3800 RPM

Changed the spark plugs and it is now back to 5500 RPM

Glad it all worked out for you. Based on your previous information, you must be doing around 30 MPH now.

 

[tchpet2]

Re: Change prop after hydrofoil?

I haven't had a chance to "borrow" my wife's GPS yet, but hopefully it is a bit faster. I had the tabs set too stiff and they were dragging through the water (lots of drag) but it seemed to be going just as quick as it was without them. So once those are set correctly, I thnk it will be doing 25-27 mph. We shall see