Novice - Boat wake / performance question.

I have a 32' power boat that I recently repowered (twin diesels). I had access to a good prop calculater and also had Michigan wheel do a full repower analysis. Upon completion, the boats performance does not meet performance predictions or expectations.

I understand most calculations are estimates and that actual results will vary but I'm trying to understand how the inputs equal the outputs. For example: One calculater, "vessel power requirements" asks for desired speed,vessel weight, and hull form (of which there are several options) . It then calculates the power required to attain this speed. In my case, I needed 392 crank hp (146Kw) to attain 20 knot cruise and 24.5 knot top speed. However, with the same inputs on the prop calculater (same software just different outputs), the predicted max vessel speed was 22.0 knots. The Michigan Wheel analysis predicted even better performance and recommended higher pitch on the props (which caused me to repitch).

I have a planing hull (modified V with relatively low dead rise (don't know the angle) and the vessel weight fully loaded is an estimated (16000#) based on the manufacturers dry weight with additions made for, fuel/gear/people etc. My vessel is only lightly loaded at this point and still falling short of the numbers. I have asked around to see if there was a way to accuratly determine boat weight (without transporting the boat to truck scales) but I haven't found anything. Certainly if my boat is 1- 2 thoudand pounds heavier than what I thought, this would explain why I'm seeing what I'm seeing. I also notice my boat creates a significantly larger wake than others of like size and weight.

My hope is to try to understand the factors that effect performance to see if, the predictions were wrong or if my vessel has another inherent problem. Any input or questions welcome/

 

A couple of points for you to consider are.
1, did any of the power calculation take into account loses due to gear boxes, shafting etc?
2, what kind of trim is she running at?
3, A large emount of wash is sometimes an indication of a heavy load plan form, without weighing the boat the only other way is to use freeboard/draught marks and hydrostatic curve information, which I assume you don't have.

what kind of speed are you getting?

 

Also, are the engines reaching the expected RPM @WOT?

 

Morgig,

thanks for the reply.

Yes, all the calculations made allowances for drive train losses (I believe 5%) and this number was consistent with Volvo's printed numbers. As far as trim goes, I don't know the running angle untrimmed, but I am able to fully adjust running angle at any speed.. in other words, the trim tabs are still effective and I haven't changed the LCG such that I can't trim her out. Your right, I dont have any any way of obtaining displacement values. I also failed to mention that I'm seeing fairly high prop slip numbers at cruise (almost 35% slip).

My current attainable maximum speed is 21.5 knots (18.5k cruise) but these numbers are with a lightly loaded vessel ( no water/waste, 5/8 full fuel tank, minimal gear and just 2 persons on board). according to my calculations, thats almost 1500# less wieght than used in the calculations. If I was getting this performance fully loaded, I think I'd be close enough not to worry, but to be falling short unloaded has me concerned.

Woodboat,

my guess would be no, but I can't confirm that at this point. I installed digital tachs running off crank triggers but the signal is failing above 3600 rpm ( I have some electrical noise issues). I plan on changing the prox sensors and have confrimed that the current ones are accurate to within 2% at cruise (3400 RPM) but anything above 3600 would be a guess at best.

 

OK so cruise target RPM is 3400 and WOT is 4000? So we have an engine that is not making enough power, a boat that is heavier than expected or a hull design that is not creating enough lift thus requiring more power to go the same speed. We also have the possibility that the prop is simply wrong. What was the previous power plant? How did they perform? What props did it have? What is the current prop specs? I must admit 21.5 seems a little slow for 400 HP in a 32 ft boat.

 

Woodboat,

correct on your conclusions... the million dollar question is how does one begin to determine which factor, or combination of factors is impacting performance.

Old power was twin chrysler 318's (225 HP). Gearing was the same 1.5 to 1. Props were previuosly 16 X 15 light cup. Vessel weight did not change much going to diesels (engines added 600# but 3 old steel tanks were replace with 1 aluminum which reduced weight by 2-300# so net gain was only 3-400#).

With the old engines, one would hit WOT (4000 RPM) but the other would only get to 3650 (if the tachs were accurate, I never calibrated them with a photo tach). Max speed was 26-27 knots lightly loaded, on a clean bottom. Cruise speed was 15-17 knots @ 3000 (just before the secondaries opened)

New setup is twin Volvo 200 hp (3400 cruise, 3800 max) with 1.5:1 gears and the props were repitched to 16x16 NO CUP. (I'm regretting not having cup but it was a compromise between one analyis that recommended 16x15 no cup and another that recommended 16x16 Medium cup). Also, I'm guessing I'm not getting to WOT based on speed becuase the tachs are just unreliable above 3600. I calculated slip using the known good 3400 RPM and speeds.

 

I am not a "prop shop" but have owned a few boats Looking at it logically the boat designer/manufacturer probably tested a few props before settling on your 16X15 Cupped. Meaning that the boats actual weight and hulls lifting ability were proven to work with that setup. Using that as a base not a computer program I would have made a few conclusions. You have added weight. You have reduced total power by 50 Horsepower. You also now have a slower max RPM. Between the increased weight and lower power my guess would have been that the original props would have put you right at the desired max RPM. So I don't know how one would expect to reduce horsepower, raise the weight and achieve virtually the same RPM with more pitch. At this point I would say you need reliable tachs and a good WOT number before you can proceed.

 

Good logic but not quite accurate. The 16x15 cupped props were not original. It was the previous owners attempt to deal with his addition of a large steel fuel tank as well as some tired engines.

The vessels original power options were either twin 225 hp 318's OR 200 hp Chrysler diesels. Both power options came from the factory with 16x16 props (but cup (or no cup) is not specified). Since Michigan wheels repower analysis called for 16x16 medium cup, I figured 16x16 uncupped would provide a safety margin so that the engines would not be overloaded.

I agree on the reliable tachs... thats in the works. Any comments on the slip and large wake?

 

From "power boating"
"Cupped propellers have an extra curve on the trailing edge of their blades, which enables the prop to cut through water better. A properly cupped propeller should give your boat a higher top speed or allow you to go faster at the same rpm's. A rough rule of thumb for cupping says that a medium cup equals 2" of pitch. In other words, a 14" x 17" un-cupped prop could be replaced with a 14" x 15" cupped propeller and produce the same engine performance with higher speed"

It is my understanding that cupping increases efficiency and reduces slip. I have always chosen cupped and adjusted pitch accordingly. The engines are either straight or Vdrives.... Either way this adds the weight behind the boats center. This alone could increase the wake. One would think if you could apply enough power it would climb up a bit more and reduce the wake. I'm still not convinced that 16X15 or even 16X14 Cupped wouldn't be a better choice regardless of what analysis said. I think I would have put a bit more power in. With that said I am sure at this point that isn't an option so I ask... what has michigan wheel said about actual perfoirmance versus their analysis? secondly you may need to add some lift. Does anyone know if these work http://www.thesmartrail.com/ The before and afters look good.


Oh is it possible that the move to 16X15's wasn't because the boat has gained some weight over the years? My dad's boat had some foam that had become completely saturated. Is it possible that you have some moisture in some bulkheads, stringers or somewhere adding some weight? After gutting my dad's boat it picked up at least 5 miles an hour. I don't think it was this fast when new in 1973.

 

I spoke to michigan wheel and the tech made 2 comments. He thought the props should have been cupped due to the shaft speed, but he also agreed that the addition of cup to the 16 pitch would have things even worst. He suggested that I compile as much data as possible I call back to speak to thier Naval Architect. To do that, I want to at least have my WOT values so I need to wait until my tachs are fixed. Also, I understand the output is only as good as the input. I was hoping there would be some way to determine my atcual weight as opposed to relying upon the 30 year old manufactures data as the base line... for all I know, that was 1000# understated.

As far as the re-ptich goes, I know what the factory spec'ed for the original props and I know I was told that the repitch was done to address the new tank and weight of fuel (120 gallon original capacity versus the addition of a 250 pound tank as well as another 170 gallons of gasoline). The story made sense but I suppose anything is possible. The hull was surveyed when I bought it and it is not cored. Stringers and bulkheads were OK and the deck, while cored, did not appear to be saturated.

 

Can you calculate, err guess your boats displacement? http://www.physicsforums.com/showthread.php?t=33706 If was briefly mentioned by morgig but is it safe to assume you don't know the numbers?