The mystery of a proper prop and terrible performance

I didn't express myself very well. Those high back helm seats weigh 40 pounds each. The dink about 70. So it's 150 pounds for all three items, not that it matters. You have me thinking though.

I know where the water line is. The boat's in the yard. I should be able, without too much difficulty, to calculate the volume of water the hull displaces. Long ago, in the late 70's I passed Calculus III. I don't remember a lot of it today but there are methods of calculating displacement when you don't have a scale. Might be a worthwhile winter project.

We'll see.

MIA

 

My pleasure, anything I can do to assist.

However, without sounding like a broken record (showing my age with that comment), i simply cannot state clear enough, you need to know the actual true weight of your vessel and where possible, depending upon how you obtain the weight, the current LCG in the lightship condition.
Everything starts knowing these values; without which it is pure speculation and assumptions that could all lead down the wrong track and wasted time/money.

 

Glad to see the update MIA.
I can certainly attest to the benefit of tabs, at least on mine. I cannot think that won't be the missing link to getting her over the hump! I definitely 2nd what Fall Guy recommends as well. Strip her bare as a test and add weight back as you go after the tabs are added.

 

If you have an accurate waterline, it is not too difficult to calculate the weight; just tedious. Divide the submerged volume into small sections and add them up. Multiply by the density of water ~64 lb/cu ft.

 

Yup. The smaller the sections the better the estimate. I remember finding the area under the curve in math class all those years ago. I'll have time to do an estimate over the winter. Based on my observation of the boat when it was recently under full throttle, I'll bet that she's not too far over 7,000 pounds. But the math will tell us.

MIA

 

I've been told by people smarter than me that it's easiest to jack it up corner at a time, and weigh it that way. I didn't get to that, because I imagined the travel lift could tell me. Nope. Serenity was too light to get a good read, but then my problem was never going to be weight, it was always going to be hydrodynamic.

 

Can't believe I started this thread three years ago. Well, my beautiful old boat Pearl will be back in her slip in a week or two. Finished the trim tabs. It will be interesting to see how she does. She'll either plane or she won't. Besides me, I don't know if anyone is interested in how this turns out. So many times people take time to respoond to questions and the OP never lets anyone know how things turn out. I'll post either way it goes. MIA

 

MIA, I've been quietly lurking here in the deep waiting to see how you made out! Can't wait for the report

 

Definitely interested over here.

 

Finally, FINALLY got out for some speed tests. Between the flooding rains, smoke and occasional heat it's been one thing after another. There's been so much debris in the Erie Canal System that I decided to wait. Finally got a decent day so I filled the fuel tanks and water. I wanted the boat to be on the heavy side. No photos on the test run because I couldn't find anyone to go out with me. Oh well.

As you'll see from last summers work my boat was, in spite of my best efforts a displacement boat. I installed a set of 24X12 hydraulic trim tabs this spring. Also used drop fins. The fins were inexpensive and I figured "why not".

I dropped the tabs about 1/2 way and increased the throttle. At 3,000 RPM the bow rise was much less so I extended the tabs to 3/4 full. The stern started to come up pretty good. I increased the throttle and watched the speed on the chart plotter. 9, 10, 12, 14 knots. The rate of increase started to slow so I raised the tabs to about 1/4 extension. 15, 16, 17.....

After four runs, two upstream and two down stream I averaged 17.6 knots speed over ground or a little over 20 MPH. Not a speed demon but she's up. The goal of all this was not to have a fast boat. I want to use this boat in the ocean and wanted the ability to get on plane when necessary when passing through inlets and areas of strong currents. If she never goes faster than about 18 knot's that should be good enough, so I guess it's mission accomplished. I'll bet I can get a little more out of her though.

Someone is going to ask about full throttle RPM. This is where I was surprised. She could only make about 3,700 RPM. Full throttle RPM is supposed to be 4,400. So it looks like there is a little propeller work to do. I'm thinking maybe dropping a bit in pitch. I'll call Michigan Wheel in the off season and see what they think. I've never gotten an accurate weight estimate on this boat. maybe she's heavier than I thought. If I burn 40 gallons of fuel out of her and dump the water tanks I'll lighten her up a bit and see what she'll do.

I want to thank everyone who took the time to comment on this long thread. Everyones imput is/was greatly appreciated. I couldn't have built this boat without Boatdesign.net and all the input and encouragement I got from it's members. My only regret is that Paul Ricelli isn't around. Can't believe it's been five years.


MIA

 

Wonderful news MIA! Here is my post from a few years back after having my prop worked: Calulated propeller efficiency https://www.boatdesign.net/threads/calulated-propeller-efficiency.61365/page-3

I had the prop worked to include a fairly decent cup. Upon review from the prop shop, the diameter was actually 16 while the prop was originally a 17.
So now, I am running a 16X16 cupped 3 blade.

Right off the bat, there appears to be a significant increase in efficiency.

I don't have the full range of speeds per RPM, however at 3000 I picked up 2 knots (13.4 to 15.4) Top end RPMs are now limited to 4250 (expected based on the cup).
However at that RPM I am now seeing 24 knots. 25 knots is not out of the question based on conditions.

One other item of note, the boat no longer needs the trim tabs as much to maintain the best speed. As I have experienced with other boats of my past, I now must bring the tabs in to hit the best speed at a given RPM conditions permitting. Prior to the prop work, the boat needed full trim tab at anything under 3400 RPM to get the most speed at that RPM. It would begin to drop off plan and slow significantly if the tabs were not extended fully.

Into the wind slightly at 3000 RPM with tabs at mid range resulted in the 2 knot increase ...

Assuming you're certain the carb is opening all the way, looking forward to the results post prop work. Enjoy the rest of the summer

 

Good observation about the carburetor. I'll have to double check that.

As long as I'm at it, I wanted to put a plug in for a company that produced a product that really made a difference. Performance Distributors down in Memphis, Tennessee. They built one of their Davis Unified Ignition distributors for me. I'm sure that the trim tabs made the difference in getting up on plane but while my engine ran fine with the old Pertronix system I could feel and hear the difference when I installed the new unit from Performance Distributors. I think it made a positive difference as well.

I'll feel a little more comfortable heading for Manasquan or Barnegat Inlet now!

Regards,

MIA

 

Manasquan is my home inlet, if you do decide to make the trip I'd love to welcome you!

 

Nice news MIA, I am happy that you managed to get on plane. Two questions - 1. What is the purpose/advantage of those dropped fins on the tabs? 2. Would such dropped fins help with keeping more stable direction at displacement speeds of (semi)planning hull (most often tabs are fully retracted/up during displacement speed)?

 

There are lots of opinions on drop fins. Do a search "Bennett trim tab drop fins" and you'll see a bunch of threads. A set of these only cost $30 with shipping. I used 1/4-20 SS machine screws to mount them. They're supposed to give you something like 30% more lift. I'll never really know unless I take them off and see the difference. A lot of people seem to think they make a difference. Like I said for $30, I figured "why not".

I've just had Pearl out once to test these tabs out so I'm not too familiar with them yet. Aside from the lift aspect I noticed one big thing. If I adjusted one tab without adjusting the other one, it felt like I was banking an airplane. I thought to myself "Hmmm, I could keep the rudder straight and steer with these things if I had to." It was a wierd sensation. At displacement speeds I didn't notice any difference but I wasn't really paying that much attention.

Even though the tabs were up at the time, I thought that backing to port was easier but I'm going to have to play around with that aspect of things. Looks like we'll get some decent weather up here finally over the coming days so I should have a little time to go play.

I'll do a follow-up when I know more.

Regards,

MIA