Prop slip issue? inboard

[csdwarf]

Hey everyone,

I will try to make this short and sweet for 30 years my father has had, and has run his 1965 21 foot trojan cabin cruiser. It has ~300hp 350 chevy in it.

the engine was rebuild and upgraded for this season. The old engine was tired and believed to be underpowered. With the old engine we were only seeing a WOT rpm of 3000. along with low RPM we were seeing a very high bow altitude.

with the new engine power isn't an issue. rpm will climb quickly to 5000rpm before WOT.

I expected with the more powerful engine that the boat would "get up over" this bow altitude issue, climb onto a better plane. This wasn't the case. The engine just revved up.

In the past I had heard of a quick and dirty test. Put a rope around the stern of the boat to provide lift. So I decided to string a piece of 3/8 rope around the stern.

The boat leveled out great, altitude was right where I would expect, and the wake looked much better. We were unable to try WOT with the rope because it would come off, but it was a successful test.

before the rope we were seeing ~12 knots @ 3000 rpm, and with the rope the boat ran much more level and almost 16 knots at 3000 RPM.

the configuration in the boat is a 350, with a 71c velvet drive 1:1, and a 13x11 3 blade prop 1" shaft.

im getting 45% slip, and that seems very wrong.

the boat is an lapstreak, plywood bottom fairly flat, 21 foot, 8' beam ~12deg shaft angle.

Is this all caused by a prop issue? The original engine was the opposite direction, but the same specs 13x11

any thoughts would be awesome.

with the rope, at a 16-18 knots cruise depending on the current, the boat is decent, but I would like it to perform much better if possible. The transom needs lifts, maybe trim tabs, or wedges on the hull?

Thanks guys!

 

[sasto]

Is your WOT at 5000? If so you appear to need a larger or more pitched prop. Ideal RPM at WOT would be around 4500. Good Luck!

 

[steelespike]

While the rope is partially successful it is introducing a lot of drag.
You need to get rid of the rope and make or buy adjustable tabs,
Smart tabs comes to mind.When applying the tabs it should be only just barely enough to help it plane.
You need more prop.I had a1929 25 ft antique launch. 4000 lbs dry weight 1.1 ratio,125 hp.
It ran a 14 X 14 prop it was a little too much. motor was rated for 3200 rpm with a lot of torque, and only made 2800-2900.
I had a 53 18 ft Chris Craft about 1800 lbs dry weight 105 hp 1.1 ratio I believe the prop was about 12" Seems it was a 12 X 12
Its wot rpm was about 3600 I guess the speed was 35 I never checked.
I had another 15 ft inboard about 1500 lbs with a 95 hp.If I remember right it was running a 11 X 11 and over revving.
Point is you have 300 + hp probably about 4000 lbs or more dry weight I'm thinking a 13 or 14" prop perhaps a 14 X 13.
You need a lot of diameter because of the weight.
You might contact Michigan wheel Give them your details.

 

[csdwarf]

What do you expect for a slip value on a boat like this? I am curious how going from a 3 blade to a 4 blade will effect slip. 45% seems like a lot.
RPM....HP...TQ
1500 87 305
2000 135 355
2500 182 383
3000 225 393
3500 262 395
4000 293 386
4500 320 375
5000 335 352
5500 332 317




Above is the dyno sim of the engine that is in the boat. It is nothing more then an educated guess, but it gives us something to go by. You said you ran a 14x14 on an engine that made 125hp @ 3200 rpm. This engine apears to capable of making much then that.

The next issue is the fact that the boat cant spin more then a 13" wheel, there isnt enough clearance to really run any bigger. There's only a few inches as is.

I have been searching for 4 blade prop, as I understand it, the added surface area of the 4th blade creates the same effect as the larger diameter.

At this point we have two variables to juggles, the fact that I am changing to a 4 blade prop, and trying to adjust the pitch to match the new engine. I have read the 200rpm/1" pitch rule as a guideline.

I have decided 4800 is a good WOT rpm to shoot for.

what do you guys think about a 13x13? then the next option I have is a 13x16, that would probably be way to much?

thanks guys!

 

[steelespike]

I think a 13 X 13 is a good start. Your slip is very high because your boat is struggling to plane, 1.1 puts more strain on the motor.
The Boat with the 14 X 14 prop was a 1929 Dodge with a Lycoming 325 cu. in straight eight.Lots of torque. The original old school prop was a 14 X 15
designed to deliver 3200 rpm and 31 mph.The 14 X 14 Michigan Wheel (much more sophisticated) was put on in the 30s and was too much
The boat was in storage for many years so the prop was never changed.Motor was within its rated rpm so a prop change wasn't really needed as the boat was
operated mostly at about 1800- 2000 rpm.
Don't worry about your slip as speed increases slip will improve. At 4500 rpm your prop will be turning g 4500 rpm.
Speaking very roughly the prop on an outboard or I/O will be turning "about" half the engine speed.
On a bass boat you might find a 200 hp turning a 24 or 25" prop. at speeds well over 50.