Under-Propped ?

[Marian Claire]

[[[Edit:The RPM #s in this post are no good. I found that my tach was way off. Corrected #s in a later post.]]]
I have discussed this issue with a few forum members but now that I have more info and a plan I wanted to post. The basic problem is I never reach hull speed, or even close, under power. I believe my hull speed works out to be 7.6 MPH with a 25 ft LWL. I will use MPH due to traveling on the ICW.
I did some test runs the other day and they are very similar to tests in the past in calm conditions clean prop etc.
1200 RPM 3.1 MPH
1400 " 3.5 "
1600 " 4.0 "
1800 " 4.5 "
2000 " 5.0 "
2200 " 5.5 "
2400 " 5.7 "
2600 " 6.0 "
2800 " 6.2 "
I have the 2 to 1 reduction gear and the engine showed no sign of strain or of topping out.
I have sailed faster than this so I do not think it is a hull speed issue. I just think I am under propped. The prop is 14X?.
From the #s, point of diminishing returns, and the 2000 to 2500 RPM continuous use recommendation for the A-4. I will shoot for 2200 as a cruising RPM with a few hundred more in reserve.
From what I understand you lose 100 RPM per 1" of added pitch. I can not go to a bigger diameter due to the MCs aperture. My plan is to add 2" of pitch to my existing prop no matter what pitch it turns out to be.
1. Does this sound reasonable?
2. What is the max a props pitch can be changed?
3. Extra Credit: What kind of performance improvement can I expect?
I have contacted several prop dealers and they have been helpful but none of the props they suggest are 14" and some are to large for the aperture. The smaller ones might be negatively impacted by the width of the keel just 5 or 6 inches forward of the prop. Plus the MC is a homemade boat so there are no boats to reference her to. I would like to modify and use my existing prop if possible. Thanks. Dan S/V Marian Claire

[Dave Neptune]

Prop

Dan, with a 14" prop you should be able to go up 2" or even a bit more. However do make sure that the prop has not been pitched up a bit already. Increasing pitch will move the "rake" of the prop in a positive direction, a good thing. This will put more of a load on the outside dia of the prop which is a good thing on an aparature app.
Is it a 2 or 3 blade?
Often with close and thick deadwood it is good to go to the larger diameter in a 2 blade and cut the dia down leavig the blades with a more square looking end. This will give you a 2 bladed prop you can still hide and the larger prop gives you wider blades especially out where they are "exposed" giving you far better traction.
I would also look at the load on the engine with a vac gage on the last 3 RPM settings. That can give good insight into how much to pitch or just go bigger and cut down.

Dave Neptune

[Marian Claire]

Knew I would forget something. It is a 2 blade. Thanks for the idea of cutting back a larger diameter. Will the prop shop be able to tell if the prop has been changed before? Dan S/V Marian Claire

[Ball Racing]

If it's a good shop, they will re- mark the new number on the hub over the old #, or cross out the old.(the old pitch number should still be there)
On most boats, even though they can all act different,, one inch of pitch or diameter makes around 200 rpm change.

[Dave Neptune]

Marks

Dan, std operating procedure is for the old number to be "X"'t out or ground off and the new pitch will be stamped on the hub or one of the faces of the hub. It is not uncommon to see 2~3 "X"'t out on old props.
Re the 15" dia you shold be able to find one on line ar at a use chandlery for a trade or about 30~40% of value if it is good shape. You can also get a prop for a larger shaft and use an adaptor, they work fine and it will increase the probability of finding something close. Remember on the style prop you are looking at only pitch up or buy a prop that has been pitched up. Walk away from anything else.
Don't disregard the engine load, it is easily measured by manifold vac at any specific RPM.

Dave Neptune

[Mo]

Link for ya

Hi Dan,
Let's look at the boat also. Do you have the calculation formula for hull speed worked out properly. Personally, my C&C 30 has a water line is 24.6 feet (I taking this from memory) and my hull speed is 6.6 kts. When I first put the boat in the water in the spring, bottom clean with fresh paint, water tanks empty and no accumulation of junk (yet) I hit 6.2 no problem. Once I start loading her up it will be 6.0 kts under the engine....this happens as soon as I load the boat (I do put allot of "junk" on though for kids, food, games, books, toys...you name it.

Hull speed for 25 foot waterline is about 6.75 kts.

http://www.cncphotoalbum.com/technical/hullspeed/hullspeed.htm

Hope this helps you out. I think I'm using a 12/6 three blade....I can double check that as the boat is now on the hard. Direct drive.

[Marian Claire]

Mo: I believe so. I get 6.7 knots and that times 1.14 statute miles/nm is how I got the 7.6 MPH. I know using MPH is goofy on a boat but that is how the ICW is marked/measured. So bridge timing, distance to next anchorage etc is easier to figure if I know my speed/potential speed in MPH. I am sure the load I carry when cruising is a factor but I read of boats with similar LWL and higher displacement, the MC is 10000lbs, getting up to hull speed with the 2 to 1 gear. The boat was relatively light for this last test run. Water tanks empty, no extra fuel etc. I do not come close to 7.6 mph even at 2800 RPM.
I hope someone can answer #3 or tell me there is no way to know. I am not looking for miracles here but even a 10% improvement would make a big difference on a long trip or crossing to the Bahamas.
BR: Even the guys in the yard could not find markings on my prop. The other two that came with the boat are marked as you describe. I will double check my info. What I have read is 1" diam = 200 rpm and 1" pitch = 100 rpm. This seems to be mostly applied when boats are over propped and they are trying to get the rpm up. I am assuming it works in reverse?? If your #s are right I will need to back of on my plan. I want to be able to hit at least 2500 rpm as that is about 20 hp for the A-4 and I have been told I need 20 hp to get to hull speed.
Dave: I do have a 15X12 prop so your idea of cutting one down may come into play. I am researching the vac gauge issue.
What rpm/vac #s would be most helpful?
I also found one cross reference to a 14" prop. A 14X10 or 14X11 is equal to a 15X8, all two blades. A 15X 8 is one of the props suggested by Robert Hess for the A-4. So I have something to aim for.

Thanks to all. Dan S/V Marian Claire

[Marian Claire]

I have the early model manifold so I would need to drill and tap it for the vac gauge. Could I avoid that by installing the spacer/adapter sold by Moyer for using a late carb on a early manifold and connecting the gauge there? I have a spacer and that would be fairly easy. Dan S/V Marian Claire

[Marian Claire]

I think BRs pitch to RPM numbers are better than mine. I can not find the 1" pitch = 100 rpm that I used. They seem to range from 1" = 150 rpm to 1" = 225 rpm. So 1" = 200 rpm sounds good. Thanks. Dan S/V Marian Claire

[joe_db]

What happens past 2800 RPM?

[Marian Claire]

I pushed her slightly past 2800 briefly and she ran fine. In hind sight I should have run it up to WOT. I just did not think I would ever run that high in regular use so why go there. I am learning as I go. My logic was if she can handle 2800+ rpm with the existing prop. Then I could increase the pitch and even if I lost a few hundred rpm so what. I would still hit the "magic" 2000 to 2500 rpm range and hopefully have a little more speed. I am beginning to realize its not that simple. Dan S/V Marian Claire

[Dave Neptune]

Example

Dan, I am a direct drive using the Indigo and my power curve is fine.
1800 RPM @ 10" of vac easy cruise .7~.75 GPH
2000 RPM @ 8+" of vac cruise @ just over .8 GPH
2200 RPM @ 7" of vac fast cruise @ just about 1 GPH
2400 RPM @ 4.5" of vac oh KRAP cruise @ about 1.3 GPH
WOT 0" vac @ 2600 rpm throttle arm about 1/2 way open any more throttle equals a bog because we are at close to 0"'s or effectively full throttle.
Most of us who use the Indigo are very pleased with what Tom has worked out with this prop. If you can hit around 4.5~5.5"s of vac at your max power cruise you will still have a bit of reserve and the engine will spin in a very happy manner. With your 2:1 advantage you should shoot for a 2800+ for your effective WOT. Once you reach around 4" of vac you can gain very little power by advancing the throttle. A good easy cruise at around 7" works very well for economy and not to much cylinder preassure at lower RPM's. I use the stock J-8's @ .035 gap (they last about 3 seasons and I change them out) and my engine is in good tune but very tired (it's been running for 43 years now & 26 with an EI) with low compression in the back 2. I have also leaned the mid range of my carb and use a fixed jet!
A good base of calculation for these A-4 flatheads is 2.2HP per pound of fuel per hour, this can very a bit up or down depending of fuel mix and tune.

Dave Neptune

[ArtJ]

Mo: I believe so. I get 6.7 knots and that times 1.14 statute miles/nm is how I got the 7.6 MPH. I know using MPH is goofy on a boat but that is how the ICW is marked/measured. So bridge timing, distance to next anchorage etc is easier to figure if I know my speed/potential speed in MPH. I am sure the load I carry when cruising is a factor but I read of boats with similar LWL and higher displacement, the MC is 10000lbs, getting up to hull speed with the 2 to 1 gear. The boat was relatively light for this last test run. Water tanks empty, no extra fuel etc. I do not come close to 7.6 mph even at 2800 RPM.
I hope someone can answer #3 or tell me there is no way to know. I am not looking for miracles here but even a 10% improvement would make a big difference on a long trip or crossing to the Bahamas.
BR: Even the guys in the yard could not find markings on my prop. The other two that came with the boat are marked as you describe. I will double check my info. What I have read is 1" diam = 200 rpm and 1" pitch = 100 rpm. This seems to be mostly applied when boats are over propped and they are trying to get the rpm up. I am assuming it works in reverse?? If your #s are right I will need to back of on my plan. I want to be able to hit at least 2500 rpm as that is about 20 hp for the A-4 and I have been told I need 20 hp to get to hull speed.
Dave: I do have a 15X12 prop so your idea of cutting one down may come into play. I am researching the vac gauge issue.
What rpm/vac #s would be most helpful?
I also found one cross reference to a 14" prop. A 14X10 or 14X11 is equal to a 15X8, all two blades. A 15X 8 is one of the props suggested by Robert Hess for the A-4. So I have something to aim for.

Thanks to all. Dan S/V Marian Claire

Hull speed =1.34 times (square root LLWL) which is 1.34 x 5 = 6.7 knots

You are already getting hull speed. Hope this clarifies things a little.
IN miles per hour the ratio is approximately 6/5 so 6.7knots = 8.04 MPH

1 nm =2024 yds or 6072 ft 1 stature mile = 5280 ft approx. 6/5 per NM

Regards Art

[hanleyclifford]

Hi Dan - Yeah, it's not an exact science for sure - kinda like bracketing an artillery target. The key thing in my experience is to move by stages from a known prop spec. A good shop can safely add 2" pitch to a prop that has not already been altered. Regards, Hanley

[Marian Claire]

Dave: Thanks for the #s. That gives me something to look for. It might be weeks before I can do another test. I guess I could do it at the dock as MPH is not needed.
Art: Sorry for the confusion. My "I get 6.7 knots" refers to my math not actual performance. Performance #s are in the first post.
HC: How is T-ville? Glad I am doing this now. With the forums help when I do haul the MC I will know how to proceed.
Still no takers for question 3? Dan S/V Marian Claire