The mystery of a proper prop and terrible performance

On more than one occasion, I have tested the ability of a twin engine boat to plane on one engine, which is important information if you want the ability to run back through a bar, with one engine out, this invariably involves using full throttle, if it didn't, it would mean the engines were under-propped. Only then can you get a guide to what props will allow the boat to plane on one engine. If it won't plane on one motor, you can try changing to lower pitch, and the engine RPM you got at full throttle, will offer some information to help that choice.

 

Thanks for posting this. I've looked around a lot for some literature from this era but I've never seen this. I'll have to print this out. This one is close to my boat but a little different. It does spec out the prop. Look at the box where the performance graphs are located (blue box) and you'll see it in the text. The spec says they used an 18D X 19P prop. This set up would have the prop hitting the bottom of my boat as I have only 18" total clearance from the shaft center line to bottom of the hull. I can't see how a 305 Chevy could swing that much prop. Maybe the gear reduction was greater. Walter Gear offered the RD-20 v-drive with reduction up to 3:1. My gear reduction is only 1.48:1. The specs for speed do match mine up to about 2500 RPM then my boat pretty much just stops.

Just a couple of keystrokes on the calculator shows a plain Jane 18" 3 blader is going to have 50%+ more developed blade area than the developed blade area of my 15 inch prop. That alone would go a long way towards solving the blade loading problem. I'll bet it could have used a 4 blader though. Interesting the article mentions trim tabs.

Just a note to Mr. Efficiency. I don't think these were that popular. I only know of three that exist in the US. Mine, one in Long Island and one out in Washington State I think. Mine was well cared for by the previous owners but nevertheless needed a complete restoration as the whole boat was rotted out save for the hull itself and the running gear. I don't know how many were built.

There is a mistake in my mental math in the paragraph above. An 18 inch diameter prop would physically fit under my boat but the clearance between the prop tips and the hull would be 1.5 inches. Not hitting the hull but nevertheless far to tight for proper operation. With an 18" prop you'd need a minimum of almost 4" of clearance (20%).

Thanks for the posts!

MIA

 

Good job by Barry to find that article, which should help.

 

The boat described in the article had an 18 x 19 prop, for the figures to stack up, it would have been using a reduction at least 2.2 or 2.3 to 1. That's a lot different from 1.48 to 1. The 26 mph top speed is very sedate, I think the 3" gap you say exists in prop clearance is telling you something, you are running a box and prop more suited to a ski boat.

 

Your example of the 18x19 inch prop running at the same boat speed on a similar boat indicates a higher numerical gear ratio and since the prop is turning at lower rotational speed you need more blade area than your faster turning lower pitch prop at similar power. This makes perfect sense. It also helps bound the problem. If you get the 17 inch diameter four blade it should work well. The selection of the higher speed gearbox allows the use of a smaller prop diameter (which incidentally matches your setup) so that they don't need a longer prop strut and higher shaft angles and lower efficiency. It all makes sense, this is not a big boat, they didn't want to have to move the motor too far forward, and they didn't want to get too much driveshaft angle, so they went with a faster spinning smaller prop to get it all to work. Then somebody cut down the prop and it all went into the toilet. Get the right prop on it and you'll be in good shape.

 

Looking at the sluggish performance of the test boat, suggests you need to have the set-up right, to operate satisfactorily, with the 302 CID V8, it is quite a heavy boat, and somewhat narrower, and deeper-veed, than I had assumed, I think someone had done a gearbox in MIA's boat, replaced it with another with a different ratio, then switched the prop, I don't think anyone would have allowed a 3" prop clearance in the original form. You might get away with it by playing with props, but I am less confident when I see it is 8' beam and 22 degrees (doesn't look it from the pics, but that was what it said)

 

How did you arrive at the 7000 displacement number?

A 3/4" solid glass hull would be really heavy.

I don't doubt your numbers or your gut telling you to not floor it. If it is shakin; something else is amiss. Prop loading up which you proved.

My guess is that sometime in the last 46 years; someone tweaked the setup to save money on fuel or a repair. If you look closely in the engine compartment; you may see evidence of changes to the drive or even more. Of course, a paint job can cover many sins, but usually not there.

 

That is in the magazine article, as well. It is a fair lump of weight for a narrow 25-26 footer.

 

I think the boat needs to be weighed by a commercial scale.

Weigh on trailer.
Weigh trailer.

 

Re your comment Post 32

"The spec says they used an 18D X 19P prop. This set up would have the prop hitting the bottom of my boat as I have only 18" total clearance from the shaft center line to bottom of the hull. "

Perhaps this is an oversight, but if you have 18 inches between the shaft center to the bottom of the hull, there is maybe 9 inches of prop to hull clearance with an 18 inch diameter prop

 

That was a mental math mistake on my part. I have 10.5 inches from shaft center line to the hull bottom. I confused radius with diameter. Thanks for pointing that out.

 

Well, you are running a 7.5" radius prop against a potential of a 9" or 18" diameter prop. So, there is a massive difference. I was once propped wrong and all the prop did was churn. I'd say you will be enjoying some experiments come better weather.

good luck

 

Wow, 42 comments, can't thank you guys enough. I'll try and address most of the questions briefly.

I'm the third owner. Original owner was meticulous with maintenance. Older man. Used her sparingly. Older members of the boat club recall that the boat "was always slow" one guy said "Walt just putted up and down the river." I have no reason to believe that any changes were made to the drive line, except for the reduction in prop diameter. 2nd owner used to work for me. He just used her as a cocktail barge. This guy was nice enough but an oil change was about the extent of his mechanical knowledge.

As part of the restoration I removed the engine and rebuilt the stringers. Learned how to do an alignment. It's a pretty tight fit in there. Could move the whole drive line aft a few inches but there is no room to go forward. I replaced the engine in exactly the position it came out. Made a jig. Since the angle of the shaft log is fixed this tells me that the boat was designed for a 16" prop. Why the prop was shaved to 15" is a mystery to me. Maybe an experiment and the original owner just left it that was. Who knows?

I have time to think about this. For now I think the best course of action is to look into trim tabs (I'd hoped to avoid that - I hate to drill more holes) and a 16" or 17" 4 bladed propeller. I'm sure Bennett will have some ideas and I did business with Deep Blue Yacht Supply when I did a new shaft/coupling/cutlass.

If I really had to I could buy myself a little more clearance between the propeller and the hull by moving the engine aft a couple of inches this would entail lowering the engine as well. Some quick measuring tells me I could gain the better part of an inch in clearance. Not too difficult but a PITA due to the time involved. Will probably install the new prop first and see how it feels.

Thanks again for the time you all spent with this.

MIA

 

Another option is that you increase the length of the shaft and keep the other for a spare.

 

Second Barry.

could it have been shortened?

Don't you have 10.5" to hull, so 1.5" clearance takes you to 18" prop?