Propellor? or engine set up issue?

[Phil303]

I'v been making a flat bottom plywood skiff, my first boat. I tested it on the water today with a new to me (therefore untested) 15hp two stroke. but came across one major issue.

As i accelerate slowly bringing the boat up to speed i get to about half revs and it feels like the propellor starts cavitating? its ok at low/slow speed but as soon as i start to load it and try and get it onto the plane it seems to release and cavitate / airate.

I experimented with trimming it on the boat but made little/no difference.

I think i have set the boat up correctly? I set the boat level on its trailer, levelled the engine cavitation plate too. The cavitation plate sits about 50mm below the main hull in the water.

on the bottom of the hull is a rubbing strip thats 20mm thick and ends at the back of the boat. The cavitation plate is therefore 30mm below that.

So, i believe i have set up the motor about right, so why would the engine slip?

I can't tell if its slipping on the shaft?

could the exhaust be sending air to the prop?

is it the prop itself perhaps? its a 9p. should i change it?

I'm not sure which avenue to pursue to correct the issue.
any help much appreciated.

 

[Scott Danforth]

to test for prop slip, draw a line across the prop and prop nut with a sharpie marker, then go boating.

 

[Bondo]

Ayuh,..... I question 2 issues I see,....

1, the motor is to deep,.... the anti-vent-plate oughta be 'bout even with the bottom of the hull,....

2, the transom looks square to the keel line,....
Most transoms lean aft by 'bout 15?,....

 

[Phil303]

thank for the tips so far.

the marker pen for prop slip sounds an easy enough test to rule out slippage.

the transom back is square with the hull. I have seen lots of boats like this though and did not think that would be an issue?
I assumed as long as the cavitation plate was parrelel with the hull it would work ok..

the transom is 15 inches and it's a short shaft mariner 15hp from 2006. it's 16 inches if you include the rubbing strip on the bottom. that is why I'm surprised it might be too deep.

is it possible the vertical transom brings the leg too close to the hull and that interfere with it? should there be a minimum distance between back edge of hull an the front of the gearcase?

it's 14ft long an is almost completely flat from 1/3 back if that makes any difference?

help appreciated guys. thanks.

 

[Sea Rider]

Another prime example in which having AV plate set at a given distance (30MM) from lower hull/keel doesn't say much as won't know once hull rises out of water at plane at which lower leg height will water flow pass by.

Most boats have angled transoms, OB needs to sit trimmed at 90?, that's fully vertical on transom to work as expected. On vertical transoms OB should sit same. Don't know if the closeness of both can develop aeration issues while on plane, should not have an issue at fast displacement speed though.

If prop is found with no hub issues, strange that achieves aeration issues if assuming that OB sitts way down, if so, should have water splashing out, towards or over transom instead. Usually a high seated OB will achieve prop aereations on choppy, wavy, windy water cond and at tight close turns on flat water at speed. Only acceptable water performance on straight calm water courses.

Happy Boating

 

[jimmbo]

Normally the anti-ventilation plate should be close to being level with the bottom of the hull, but the keel extending all the way to the back is an issue. It should end in a taper at least a couple of feet before the transom. Then you will be able to mount the engine higher. As mentioned earlier the angle of the transom is an issue, perhaps you can utilize wedges to provide more suitable angle
A flat bottom with a fairly hard chine may have a tendency to trip in turns at planning speeds

 

[Sea Rider]

To begin seeing the light at the end of the tunnel, firstly need to diagnose if current prop is shot or not, all other points as : Proper OB height, transom angle, bottom shape, extended keel guard will not enter the equation if prop is found shot.

Assume you know how to test a prop if hub has slip issues, right ? If not just ask. Your prop seems to be starting to have its Waterloo time, it's not completely shot, but on its way to become so..

Happy Boating

 

[Phil303]

No Title

its a bit of pain in the backside to launch and give it a quick spin to test if it was slipping, our tides are not favourable right now, so i just took the prop off and had a look to see if i could see anything.

It looks a bit degraded around the rubber bush and more alarmingly seems to be off centre? i also thought isn't there supposed to be some sort of exhaust ring on the back of the propeller?

I also noted my anode seems to have come off.

So, it all seems to be bad news there. New propellor? or can these rubber bushes be fixed / replaced as the blades seem ok?

 

[jimmbo]

The hub is certainly suspect. Looking at the first pic, the blade at the top looks bent, but it might just angle, focal length of the lens and distance between the lens and the prop.

It appears the trim tab is missing

 

[ahicks]

Note the difference in the thickness of the rubber hub material one side vs. the other in the second pic. Maybe my imagination, or the pic, but that prop looks like it needs to be retired. It's shot!

Oh, and that shaft should never be rusty. If that's allowed to build up, getting a prop off can be a terrible ordeal. The prop should come off at least once a year to make sure there is no fishing line wrapped around the shaft in front of it, trying to tear up that seal, and to allow grease to be applied to the splines of that shaft.

 

[Texasmark]

I read others comments and originally felt that the transom being at 90 degrees could be part of your problem till I saw your pictures and it's ok as is. AV plate is parallel to bottom. On too low, I don't see that as an issue.....actually the opposite, being lower it's less likely to pick up surface air and hull disturbances which allows air to enter the prop wash, reducing the density and allowing the load on the prop to decrease, decreasing speed and allowing the engine to over rev.

On most boats with a keel, the keel tapers off prior to the transom reducing that potential turbulence. Keel turbulence is a problem on high mounted engines and tight turns at high speed. Where you are you seem to be far enough below that to not be a problem. If you were too low and had water splashing up on you or into the boat you would surely have mentioned that. So I see that as a non issue.

The side keels causing turbulence (bubbles) shouldn't be a problem for you (prop ventilation, not cavitation) but may if in a high speed tight turn the likes of which I doubt with a 15 hp engine.

Otherwise I suspect your next move is to do as Scott said and draw a line across your prop from the spline to the outer shell and run it. If the line moves, rehub or renew the prop. You should be fine from there.

When I was a teen I too built a wooden boat out of Oak frames and Mahogany plywood. It was quite an adventure as my finances, tools, and knowledge were minimal. I did have to change the design from a semi V to a John type construction as I had no wood clamps and couldn't get the plywood to curve up in the bow to suit my frame....so I just sawed it off square with about a 20 degree forward slope and that was that. I did use it for awhile and it worked ok with a 1956 18 Evinrude I bought new ($400) with my summer's work money. It worked ok. Nothing stellar but it worked. Can't remember what I did with it eventually.

HTH,
Mark

 

[Sea Rider]

That prop is SHOT/KAPUTT go for a new one, clean prop shaft and grease it well with marine grease prior installing a new prop. As you haven't tached current prop when new as usually loaded won't know if it's OB runs at mim-max wot rpm range factory stated. So the dilemma is : buy same one or maximize with less pitch to pull max wot revs up if in need, which you don't know anything about. A less pitch prop will account for carrying better more load or 1 extra boating mate.

You can lessen a bit the water drag under hull by rounding the square portion of front/rear and lateral borders of 3 wooden stringers seen on pic. Rounded borders are preferred to square ones under hulls.



According to OB's Owner's Manual, manual trim OB's must be trimmed vertical at 90?on transom to work as expected while combo rides parallel to water level once on plane.

The only possible way to "determine" if OB sits at the sweet OB/transom height is by visual inspection, Once combo is on plane with well balanced deck, pull head out next to OB side and check at which lower leg height is water flow is passing by through front lower leg.



Raise/lower OB on transom accordingly till water flow at speed skims right under the protruding front edge of upper plate as seen on modified pic. Post results after installing a new prop, not before though..

Happy Boating

 

[Phil303]

fitted the new propeller and took it out for a blast today. it goes like a rocket !!

pretty much achieved what I planned. planes early. shallow draft. small engine. good load carrying.

solo up it was flying... didn't dare go full throttle. 3/4 was plenty.

but with a full crew of 5 adults it planed too, albeit not rocket speed.

Will see if I can fine tune the mounted height to get the best of it, but we're very much in the right zone.

thanks for all the tips.

 

[Texasmark]

Glad you took the initiative to change the prop and thanks for the feedback. It does a lot for us folks trying to solve other's problems sight unseen.

 

[Sea Rider]

Glad you took the initiative to change the prop and thanks for the feedback. It does a lot for us folks trying to solve other's problems sight unseen.

Yep, "initiative" is key word if in pursuit of achieving top boating performance & fun as Phil in his own experience has shown.

Happy Boating

 

[Sea Rider]

Phil,

Just for the record, your new prop was exact same as previous shot one or went 1 pitch down ?

Happy Boating