The mystery of a proper prop and terrible performance

Thanks again for the encouragement gentleman. Those Interceptors are an interesting idea. If I had a faster boat I'd consider them. As it is, I think I'll go with Bennett's recommendation of the 24 x 12 tabs and see how it works out. I found a prop made in Taiwan by a firm called Hung Shen. They make a 17" 4 bladed prop pitched all the way down to 10". I'm going to check my figures one last time. As it is I'm on the fence between 10" or 11" pitch. It'll be a while but I'll let you know.

 

An old “trick” to help get a slow boat on plane was to stretch a rope tightly across near the transom under the hull.
This is probably where the inspiration for the interceptor came from.
Might be an interesting experiment for the OP, at no cost.

 

For those wishing to learn more about Interceptors, the attached old sales flyer may assist:

 

Are they that much more expensive?
Especially considering the modifications required to the hull, compared to a simple transom mounted plate which does not require major surgery.

 

I spent the day yesterday rechecking my math to make sure that I had the correct figures. I can see why prop manufacturers use software to make these calculations. Using a scientific calculator and working out the equations my figures checked out. Then I looked at the specs in that article from Boating that Barry was good enough to post (#28). How the hell did they swing an 18' diameter by 19" pitch prop with a 220 HP engine just like mine?

Here's what I think happened.

Mr. Efficiency made the right call and I want to thank him for pointing out the reduction ratio aspect of all this. For whatever reason, Silverton decided to install a 1.5 : 1 v-drive in this rig. I have no reason to believe that it's not the original v-drive. Based on the article Barry posted my boat is almost identical to the 26 footer mentioned in the article. The test boat is 7 inches longer than my boat. Not much, but perhaps Silverton tweaked the design after they built my boat in order to get the right running gear in the boat. If I'd been them I'd have thought "lets stretch her out a few inches, adjust the drive line and get the proper v-drive and prop under her." We'll never know for sure. Really doesn't matter, it is what it is.

If I leave the current 1.5 : 1 reduction v-drive in her I can go to a 17" prop but I'll need to reduce the pitch to 10 or 11 inches. Hung Shen Propellers makes a 17" x 10" prop so it's doable. It's the most economical solution but it just doesn't feel right to me. Feels like I'm putting a band-aid on it.

I spoke with the folks at Walter Gear yesterday. They make the v-drives. I can pick up a new v-drive with a ratio of 2:1 for $2,600. The newer drive will be quieter and, of course 47 years younger.

I'll have to ponder this for a bit.

Trim tabs are a must so they'll be installed first. I'm thinking of replacing the 47 year old v-drive with a new 2:1 reduction unit. Then, I can squeeze a 17" prop under her and have it pitched properly. I'll need to work out the equations but off the top of my head probably a 17" X 17" or 17" X 18". Much better solution than a 10" pitch wheel but then $2,600 buys a lot of fuel even at today's prices.

Mystery solved. Sort of.....

Thanks again to all who helped with this!

MIA

 

I don't think that you will be too happy with the performance of a 17X10 prop on this boat. I didn't read everything posted here, but I suspect that you are going with the bigger diameter to gain blade area to avoid thrust breakdown. The loading numbers do look really high, but by going to a 17" prop with more DAR the loading will come to down but still not to an acceptable level and the tip speed will be really high. That combined with reduced tip clearance may make vibration unacceptable. I would stay with the approximate parameters of the current prop and use increased blade area and cup to avoid thrust breakdown.

 

Thanks for the feedback and I agree with you about the 17 x 10 prop. This thread's been quiet for a few days and during that time I've been running numbers doing multiple Beta rho - delta propeller calculations.

I've checked my math (and it's pretty complex) three times just to be sure. What I've determined is that my boat is equipped with the wrong reduction gear. I'm the third owner. I knew the second owner but the first passed away before I decided to restore this boat back in 2007. I'm not sure whether the boat came this way or whether someone made a mistake and replaced the original v-drive. Anyway, this girl is equipped with a 1.5 : 1 v-drive and the 15x14 prop. The 15x14 is right for this reduction but the reduction gear is all wrong for the boat.

I'll be calling Walter Gear in New Jersey this week. All the math says that I need to replace the current v-drive with a 2:1 reduction unit. With a 2:1 reduction shaft RPM drops considerably. Now I can swing a 17 x 18 4 bladed prop. It happens that Michigan Wheel makes a prop called the DQ486. I'll be putting in a call to them as well. Here's what they say about that prop:

The 4 blade DQ486 comes into consideration when there is requirement for more blade area than offered by the DQ469, due to high blade loading or diameter restriction. The increased skew also helps minimize noise and vibration even with limited tip clearance, a common situation on today's performance demanding recreational boats.

With an expanded area ratio of 86 this prop drops my blade loading at 100% shaft RPM and power down into the 5 PSI area. Well below my calculated maximum loading of about 9.8 PSI.

It's going to cost me somewhere in the area of $4,000 to redo the gearing and prop but I've come this far.........then there is the trim tab issue.

Thanks again for the thoughts,

MIA

 

Maybe delay the tabs till the new transmission/propeller is tested ?

 

I'm not sure what you are anticipating for max speed, but I think that the 5 PSI estimate is too low. I calculate about 12 PSI which may be OK, but is close to where thrust breakdown may begin. The DQ props are constant pitch and while they have some skew, the sections are ogival and not very good WRT cavitation. I would still suggest some cup, and perhaps back off on the pitch an inch or so.

 

Right again Mr. E. This is getting to be a habit with you. Thanks! A week ago I thought I'd do the tabs first but now I'm thinking last. I'm pretty sure they will be needed though.

Johnneck, This isn't a fast boat. Read my first post and you'll see I only expect about 26 knots at best. I tend to run at a much more leisurely pace. Maybe 8-9 knots. I can do that with the existing 15x14 prop. The reason I want to get this boat on plane is for safety. I've been river boating and on Lake Ontario for the past 6 years. She's fine in those conditions. I'm heading for the ocean in 2021. I'll be dealing with tides and inlets. I want the ability to run faster for safety in strong currents. I don't intend to run fast often or for long but I want the capability.

I'll recheck the prop loading. I used the formula from Dave Gerr's propeller handbook to make this calculation, this is for the Michigan Wheel DQ486:

PSI = (326 (a constant) x 200 (shaft HP) x 0.60 calculated prop efficiency) / 25 (speed of advance) X 182 square inches (blade area of propeller in square inches)

PSI = (326 x 200 x 0.60) / (25 x 182)

PSI = 8.6 (this work's since it's lower than the maximum calculated loading of 9.8 PSI)

I've looked at this prop sizing three ways.

Crouches Formula tells me 17 x 18
Beta rho - delta tells me 17 x 18

Then I decided to check these figures against an online calculator with a good reputation from Vicprop.com. They say 17 x 17.8. Pretty close.
I didn't look at cupping because my reading tells me that cupping is generally preferable for high speed boats. I figured 26 knots wasn't very fast.

My calculated prop loading with this prop is well under the 9.8 PSI allowable prop loading that I've calculated. I'm going to run the numbers again with a .69 EAR 4 bladed prop and see if that will work. The efficiency of a .69 EAR prop will be greater than a .86 EAR prop so maybe this would be a better option if I can stay under 9.8 PSI. I don't know yet.

NOTE: With the DQ469 17" prop loading works out to 10 PSI. Either prop would work.

Thanks again for the replies guys,

MIA
Edited for a mistake in blade area in the formula above and the info on the DQ469.

 

282? seems like EAR should be more like 195. I don't use the empirical formulas, so I can't comment on the validity of the results.

 

Oops....This is why I check my math. The number 282 is incorrect. The blade area is 182 square inches. I corrected the post above and thank you for pointing that out.

MIA

 

Hmm, seems like that would be 1.25 EAR

 

Hi John,

1.25 would be the area of the circle, 227 sq inches divided by the expanded area of the blades.

I come up with an EAR of about 0.8 by dividing the expanded area of the blades 182 sq inches by the area of the circle 227 sq inches.

 

At $4,000 to redo the gears and propeller, plus you labor, have you considered a supercavitating propeller?