Old boat with factory prop slipping

[CaneCutter79]

I have a 1979 Cane Cutter modified tri-hull style boat. (see photo)

The current prop is a 19-pitch three blade OMC factory steel prop. I have a 17-pitch vented aluminum prop that has a bit more bow lift but not much and I loose about 3-4mph but gain a slight increase on RPMS. INCREADIBLE hole shot. I'm on plane in seconds with the small prop.

What I'm looking for is more bow lift and increased top-end WOT speed. The current speed is 39-40mph (GPS) and the motor is a 1979 85hp Evinrude V4.

My current problem is the boat runs almost level. It feels like there is too much bow and hull still left in the water at WOT. The motor will not trim up to where the cowling is at least level or higher than level without slipping. The motor sounds like its exhaust is cavitating and the mph drops from 39-35mph. Trim the motor back down and the cavitation noise goes away and the boat surges back up to 38 and then 39mph.

Also, if you leave the trim setting at maximum trim, slow to about half throttle and then hammer the throttle, the boat will rise (normal due to trim) and just before it get's back on plane the prop slips completely and cavitates. The RPMs go really high and it totally looses grip. I've also noticed that the motor has NO rooster tail. Is that a characteristic of an older boat hull? I thought it would at least have a rooster tail no higher than the cowling. I have none. Is that OK?

I do not know exactly what RPMs I'm turning but based on online videos I have seen of the same motor, everyone else is running around the 5,000 RPM range and so am I. It's not a really high pitched RPM range and it almost sounds like I have at least 300-400 RPM remaining to turn.

Do I need to swap to a 4-blade 16-pitch with a lot of rake to lift the bow more or swap to a 17-pitch 3-blade with lots of rake to lift the bow?

 

[Sandhammaren05]

Re: Old boat with factory prop slipping

First, get rid of that lumbering apparatus on the cavitation plate, that drives the bow down! Were you running with the cavitation plate way above the bottom, as on a raceboat or bass boat, that would be like an afterplane that helps prevent the boat from blowing over backward when the bow rides very high. Removing that 'excessive drag apparatus' may eliminate your problem.

If the problem remains (and leave that big plate off the cavitation plate!), then what transom height are you running? Any offset? Is the current prop cupped? Roostertail means wasted thrust, represents drag. Fast outboards that are set up right have no roostertail.

If the prop is well-enough cupped, then get a 3-4' (preferably 4') level (or other straightedge, metal machined to within .001" flat) and a tape measure and crawl under the transom. How much hook is there from the transom to 3-4' feet forward parallel to the 'kiel line' on the running surfaces (hopefully, not more than a small fraction of an inch)? With a big hook (like a sizable fraction of an inch or more) you can't pry the bow off the water, more hp only drives the bow deeper. Most boats are built with a small hook in the form of a wedge that runs 1-3" forward from the transom. With enough power, a wedge helps to lift the tail (while dropping the bow).

A boat with a perfectly straight bottom (like a V-bottom pad bassboat or raceboat) may run fine with a light motor, but with a heavier motor of comparable power may need a wedge to get the tail up. I.e., a boat that runs well with a 2-stroke motor may need a wedge to lift the tail if an elephantine 4-stroke of same hp and gear ration is used.

I have a 1979 Cane Cutter modified tri-hull style boat. (see photo)

The current prop is a 19-pitch three blade OMC factory steel prop. I have a 17-pitch vented aluminum prop that has a bit more bow lift but not much and I loose about 3-4mph but gain a slight increase on RPMS. INCREADIBLE hole shot. I'm on plane in seconds with the small prop.

What I'm looking for is more bow lift and increased top-end WOT speed. The current speed is 39-40mph (GPS) and the motor is a 1979 85hp Evinrude V4.

My current problem is the boat runs almost level. It feels like there is too much bow and hull still left in the water at WOT. The motor will not trim up to where the cowling is at least level or higher than level without slipping. The motor sounds like its exhaust is cavitating and the mph drops from 39-35mph. Trim the motor back down and the cavitation noise goes away and the boat surges back up to 38 and then 39mph.

Also, if you leave the trim setting at maximum trim, slow to about half throttle and then hammer the throttle, the boat will rise (normal due to trim) and just before it get's back on plane the prop slips completely and cavitates. The RPMs go really high and it totally looses grip. I've also noticed that the motor has NO rooster tail. Is that a characteristic of an older boat hull? I thought it would at least have a rooster tail no higher than the cowling. I have none. Is that OK?

I do not know exactly what RPMs I'm turning but based on online videos I have seen of the same motor, everyone else is running around the 5,000 RPM range and so am I. It's not a really high pitched RPM range and it almost sounds like I have at least 300-400 RPM remaining to turn.

Do I need to swap to a 4-blade 16-pitch with a lot of rake to lift the bow more or swap to a 17-pitch 3-blade with lots of rake to lift the bow?

 

[hwsiii]

Re: Old boat with factory prop slipping

Canecutter, will you take a picture of the back of the boat down level with the bottom of the boat so we can see exactly what type of bottom you have at the stern, and exactly what is the length and weight of the boat bare with nothing on it.

H

 

[CaneCutter79]

Re: Old boat with factory prop slipping

Canecutter, will you take a picture of the back of the boat down level with the bottom of the boat so we can see exactly what type of bottom you have at the stern, and exactly what is the length and weight of the boat bare with nothing on it.
H

I've tried taking off the hydrofoil. When on plane, the hydrofoil is raised out of the water so it's not dragging but it really helps for getting the boat on plane. The only difference I noticed without the hydrofoil was it takes longer to get on plane. GPS speed was no different. That's when I noticed that the hydrofoil is completely out of the water. The prop is just below the plane surface.
From what I have read online in various threads and other forums, I have a heavy little boat with a little bit of motor and 39mph GPS may be all I can squeeze out of it. It just feels like it's a little heavy in the water and a lot of bow is still dragging. If I can increase the RPM range a little and gain some bow lift from a prop with more cup, I think i will gain a little more speed and better fuel efficiency at a lower RPM or WOT.
Here are the pics

 

[Sandhammaren05]

Re: Old boat with factory prop slipping

Motor height looks good, prop is cupped too little (too small an angle, and cup doesn't go out to maximum radius). Measure the maximum radius, must be cupped more out to max. diameter (let cup taper to zero fast at max, radius). But don't go beyond max. radius with the cup. A good hammer and a trailer hitch ball will do (wear leather gloves). The existing cup must be increased a bit as well. Doing that will increase the speed and help to lift the bow. Leave the hamburger helper (bolt on cavitation plate) off and give the bow a chance to come up. Water from the prop and gearcase hits the plate and pushes the bow down.

 

[CaneCutter79]

Re: Old boat with factory prop slipping

Motor height looks good, prop is cupped too little (too small an angle, and cup doesn't go out to maximum radius). Measure the maximum radius, must be cupped more out to max. diameter (let cup taper to zero fast at max, radius). But don't go beyond max. radius with the cup. A good hammer and a trailer hitch ball will do (wear leather gloves). The existing cup must be increased a bit as well. Doing that will increase the speed and help to lift the bow. Leave the hamburger helper (bolt on cavitation plate) off and give the bow a chance to come up. Water from the prop and gearcase hits the plate and pushes the bow down.

Define "maximum radius" and how do I measure it?

 

[CaneCutter79]

Re: Old boat with factory prop slipping

So if I want to try and modify the cup myself, how do I do this? The post above mentions measurements but I'm not quite sure I follow.

The prop is a 13-1/4" dia so if I increase the cup, the prop diameter does not increase right?

 

[sschefer]

Re: Old boat with factory prop slipping

The problem that's obvious to me is the effect of the cavitation on that prop. Unless you continually run it in the rocks and sand that cavitation damage along the edges of the prop is reason alone to just replace it. As for cupping the prop yourself, it wouldn't be something that I would do.

What I would do is get rid of the whale tail, drop the engine down so the anti-ventalation plate is even with the bottom of the hull (or instal a Jack Plate) and re-prop with an inexpensive aluminum prop that is manufacturer recommended for the engine. Just off the cuff I'd say a 13 1/4 x 17 is about max for that motor in a 3 blade and drop an inch of pitch if you go to 4 to get the extra lift you want.

Unless that 19 has some special meaning to you I would dump it. I tried to straighten one and thought I did a great job until I ran it and it cavitated so badly that it blew all the paint off it.

 

[CaneCutter79]

Re: Old boat with factory prop slipping

Everyone tells me to "loose the whale tail" and I've tried both the 17 and the 19 with and without. Since I do not have a tach, I can only measure GPS speeds at this point and time.

The odd thing is, I can get 37-38mph WOT with both props without the fins (whale tail). WITH the fins on, I can get 39-40 with the 19.

Also, if I slow down to half throttle and make a sharp turn (like I was turning to get a downed skiier), if I hit the throttle hard, the props spins-out with no traction.

Cavitation and too much positive trim is my obvious problem. I cannot lower the motor as it is at it's lowest setting. A jackplate is not really an option.

It would appear that a very high performance 3-blade with an aggressive rake and cup is my best option for top-end speed and maximum efficiency. Depending on slip percentage and RPM range, that will determine my pitch.

What I need first is a tach to measure my RPMs to better determine where to go from here.

 

[Glastron1987]

Re: Old boat with factory prop slipping

If you could find a 17 inch stainless steel prop to borrow and try that may be worth a shot. Stainless bites better in the water. I believe what you say about the hydrafoil. I have an SE Sport that I really like; it helps takeoffs a lot and even reduces some cavitaton. Recently took it off temporarily, but put it right back on again.