Need help determining best prop size please

Hi all,

I have a wilson flyer (UK) 17ft cathedral hull (tri-hull) day boat which i use for fishing inshore waters of the bristol channel (UK). It has a 55hp Yamaha 2 stroke engine which is fairly old but still runs fine.

The boat itself used to have a inboard engine and a jet drive inside and through its lifetime this has been removed and an outboard has been stuck on the back. which has never been setup correctly and so the engine doesn't perform as it should.

I have tried to fix this problem before but weather conditions and difficulties has stopped me in my tracks so any help would be very appreciated.

when we brought the boat the prop was the wrong size and was just straining the engine. I think it was a 10 1/2 x 17 which under rev'd.

I brought a couple of props last year but like i say haven't really had a chance to try. The current prop is a 11 3/4 x 12 which i brought purely because it was from a 40hp yamaha.

i have brought 2 tacho but cant seem to setup them up properly

The performance with this prop is that it still under rev's.

i think the diameter is too large and i think this could be one cause of the problem.

Being that it is over reving, would you suggest trying a smaller pitch prop.

many thanks in advance and i can give you as much info as possible to help.

Re: Need help determining best prop size please

Bud, welcome to Iboats prop forum and we will try to help you as much as we can but without more information no one can help you, and I will tell you I need more information than anyone else. But if you would like my help fill this form out and I will help you as much as I can, if you would prefer to not go to this much trouble there are other people who will help you.

Iboats Boat and Motor Info
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 recommended HP for your boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. 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 and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil, dolefin or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. 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. If you do not have a tach you can buy a Tiny Tach for $ 50
14. Are you at sea level or a higher elevation, give us the elevation in feet
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the foregoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. 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
19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.

REMEMBER, The numbers I give you will be NO better than the information you give me

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

Re: Need help determining best prop size please

1. unknown (no plate)
2. 17ft long, 6ft beam, unknown weight (no plate)
2a. unknown (no plate) I rebuilt the transom which would now accommodate a larger engine
3. 2-3 people, 10 gallons max
4. fishing
5. catheldral hull or tri-hull (search wilson flyer on google)
6. European 1974-1979 yamaha 55AE longshaft outboard
7. 55hp (will try to find out ratio but will be hard for me to find)
8. unknown (unable to find a manual)
9. The cavitation plate has been dropped so it is inline with the keel (been through this process before)
10. as before it is a wilson flyer (search on google)
11. Has doelfins fitted
12. unknown make model but currently 11 3/4 x 12 alu
13. unknown (have brought 2 yamaha tachs but cannot setup correctly poles etc.)
14. sea level
15. all working fine, just serviced
16. 2 trips, brought to test because it was from a yamaha 40hp
17. none
18. the engine does not get to full revs, the less people the more it revs and sometimes when you get on top of a swell you can hear that the engine wants to give a bit more but its sluggish. I have read in many pleaces that under reving is casued by being over propped and i beleive this is true in my case. The prop which was on it before i brought it was a mercury prop and smaller in diameter (dont ask what size because i do not know)
19. very hard to do and i will explain why.

we keep the boat on a trailer and have to tow it 8 miles to where we launch from each time we go out. The place where we live is very limited as to where you can launch and at what time. We live where there is a tidal rise and fall of 10 meters a day. The place we launch is a natural harbour and is near enough level so we have to literally "float" the boat off its trailer which is the hardest part especailly with a incoming or outgoing 10meter tide ! So when we do go out we try and make the most of it before we have to get back in. The harbour is hard and the top and soft sand at the bottom which make boat recovery impossible so you can only launch and recover at the top of the tide (hard to understand as many places not really have a tidal rise and fall, especially 10 meters)

Im going to try and sort the yamaha tacho out but i have tried before and got nowhere with it. It has 4, 6 or 12 poles selection at rear. when selected at 4 and 6 it maxes out and when selected at 12 it hovers at 1 which may be wrong wires connected but i'll see.

I would buy any second hand yamaha prop just to try and get the best results.

Like i say at the moment the engine is definatly under reving and i would like to know is it best to go with a smaller pitch or smaller diameter or a combination of both.

Would like to know even if just to compare to the prop i have know, At least then i have an idea of which way i should be heading.

I will try and dig out some pictures of my boat which are on an old broken pc.

Re: Need help determining best prop size please

Bud, this is how you find the gear ratio:
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 and turn the motor until the tape lines up with each other, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor.
You also need a working tachometer in order to make any sense of what is happening with these props that you are using, if you have to get a mechanic to hook up your tachometer.
When you have these fixed then get us some numbers of WOT RPM and speed with two of the props and we can help you.


H

Re: Need help determining best prop size please

Hi,

thanks again for your reply. I will try and get the info on gear ratio asap.

I have uploaded some photos of the boat, engine and wiring loom.

http://www.easy-cb.com/boat/

i have now got the tacho working, there was a break inside one of the connecting cables which was not visable. So i will be able to get rpm's however not until we go out again which is all dependant on the weather and work but hopefully as soon as possible.

Im not quite understanding the gear ratio method.
Lets gets this straight. place a bit of tape across the mid section of the prop and turn the engine flywheel 1 complete turn and see how much or how many times the prop rotates while in gear ?

Is there any other way of finding out the gear ratio. I was going to ring up yamaha and see if they can tell me any more information on the engine as i cant find any information that i need on the net. Would it be stamped somewhere on the gearbox ? I would drop the gearbox off if i knew it was stamped on it.

budbecks

Re: Need help determining best prop size please

Just tried working out the gear ratio.

For 1 full rotation of the prop i turned the flywheel just under 1-3/4 (1.75) turns. Would say about 1.70 as decimal.

does this sound about right ?

Re: Need help determining best prop size please

Bud, I woild think that 1.71would be close, now all we need are some numbers for WOT RPM and speed. Try the boat with both propellers so we can see the differences between the two.


H

Re: Need help determining best prop size please

Hi again.


Took the boat out today and have some results.

Prop 1 - 11-3/4 x 12

WOT best @ 3600

prop 2 - 12-1/5 x 8

WOT best @ 4200 (fit 4500 once dead flat)


What you need to know, The boat is as described on my first post and in the pictures. We had 3 people on board and there will always be a minimum of 3 on board as we need 3 to launch it. 40lt fuel some fishing gear but it doesn't weigh a thing.

We had to rebuild the transom and rear deck as the old one was shot and we built it good using 2 sheets of 1" thick marine ply and th deck is made out of 12mm marine ply as to are the gunnels at the rear.

So to sum it up a bit, our boathas a transom which would take a much larger engine than before but is also much heavier than originally built and i think this is the downfall.

When i fitted the second prop (8 pitch), the rpm was higher and the engine was more responsive and she did go a little faster but it's not planing like it should and i think engine needs to be bigger.

Would love to know if i could get a different prop but the one i have now which is a 12-1/2 diameter is level if not proud of the very bottom of the gear casing (prop shield, v part at bottom) and i wouldn't like to put a larger diameter blade on.

Hope you can make something of this even if its advice.

Many thanks in advance.

John

Re: Need help determining best prop size please

John what were the speeds at those RPM.


H

Re: Need help determining best prop size please

hi sorry

7mph and 11mph repsectively

Re: Need help determining best prop size please

Ok, we are moving in the wrong diretion in my opinion. We need to put the old 10 1/2 x 17" on it and give me the WOT RPM and speed with that prop. No matter what we do this is not going to be a fast boat, so we will do our best to get the maximum performane out of the motor you have.
I was looking at the pitures you have but I an't tell if you an move the motor up any, we need to move the motor up two inhes above where it is now. No matter if you even have to drill more holes, but just make sure you an drop it down one more inh if we have to. Please take a piture like the one I am sending you so I an see how high the anti ventilation plate is above the keel.

Anti Ventilation Plate



When I get the piture we an talk about it some more.

The prop slip we are showing is between 40 to 70% prop slip right now, and that is as bad as it gets ever. So we have to redue that tremendously. Here are the numbers.

Prop Slip




H

Re: Need help determining best prop size please

I have to wonder if we are running on all cylinders or the throttle is opening all the way.The boat does appear to be a brute but a 8" pitch should scream.
rpm.I think I would remove the foil until we see what we can get out of
the motor. If its a 3 cylinder you might not notice missing a cylinder except for the performance.In fact some folks don't notice a 2 cylinder running on one except for performance.
Experiencing 70% slip yet it won't rev with an 8" pitch doesn't seem right to me.

Re: Need help determining best prop size please

Spike that makes exellent sense to me. Bud you need to hek that out. I just do so many everyday I don't always think everything all the way through Spike, and you always do, that is why you are very good at this.


H

Re: Need help determining best prop size please

Hi again.

When the transom was rebuilt it was rebuilt to the same as it was when i recevied the boat and the engine was mounted exactly how it was when i got it.

We have had this problem with this boat not going very fast or not rev'n very fast since the moment we got it. sod's law.

A few people said to me that the engine could be dropped a couple of inches so the caviation is inline with the bottom of the keel which it now is and since we have dropped the engine it HAS run better and we have seen an improvement in performance.

The other day was the first time we tried the 8 pitch prop with the engine lowered and was the best performance we have had to date.

The boat was originally built with an inboard engine and a jet drive and because of this the hull isn't as it was designed originally and so this may have an effect on the movement of water under the boat. To add to this, this is why the engine has been dropped so the cavitation plate is where the bottom of the keel should be.

Hope you can understand that.

But as i said the boat is heavy especially with 3 people on board fuel etc. We did get to 4500 revs with 3 onboard and i dont know if it would go any faster, but im assuming it would, with just me onboard. It mattered a lot as to where people were located when the boat was moving. 1 up front kept the front down but never got onto plane. 2 at back weighed the back down so much that she wouldn't plane either. If we were all sort of stood in the middle then she would plane and pus quite well.

We also have a kill handle attatched to the choke to kill the engine if needed which needed to be pushed in which open up the throttle more so the engine would run better. but the spring attached to it pushed it back so it obviously isn't how it is intended to work.

I would go ahead and put the 17 pitch back on but as it doesn't lift the boat onto the plane it will never rev high enough.

I am willing to try anything but i have tried a lot with this engine and its running ok at the moment, being not as well as it should but still well. I dont really want to go backwards if you can understand. Time is very limited when we go out and we have to make the most of it as the british weather is not very nice to us !!!

Thanks again for all your help and any more help i would be very grateful for.

John

Re: Need help determining best prop size please

John, I don't really understand what this statement means, please try to make the statement more easily understood, beause this ould be the problem.

We also have a kill handle attatched to the choke to kill the engine if needed which needed to be pushed in which open up the throttle more so the engine would run better. but the spring attached to it pushed it back so it obviously isn't how it is intended to work.


H