Engine RPMs vs. Hull Speed

[garylinger]

I need the experts to weigh in! I did my sea trial yesterday for the new Beta 16. It is supposed to reach maximum at 3600 rpm. It is in my 30 foot S2 weighing 9800 lbs. with a theoretical hull speed of 6.5. Yesterday I reached hull speed of 7 give or take a little (based on gps at various angles to the tidal movement) but max rpm I could get from the engine was 2900. In neutral it will exceed the 3600 spec. I have a 13 inch 2 bladed prop (original equipment). Can I assume that my speed vs. rpm is normal or should I be able to achieve the additional rpm? I am satisfied with the performance but want to make sure that everything is operating as it should and I am not missing something.

 

[Roger Long]

I need the experts to weigh in!

It sounds like you have too much prop for the engine but that is rare for a sailboat. If the prop is too large, it will be very hard on the engine and shorten its life, much like driving your car around in too low a gear.

Is this a re-power? You should be sure you don't have a fuel line restriction. I changed out an original fuel line and picked up 1000 RPM due to some restriction that had developed. Have you checked your filters?

An important clue is the color of the exhaust smoke and how it changes as you approach the maximum RPM the engine will develop with the boat underway.

We need about half a typewritten page of information that you haven't provided to give you any useful advice though.

 

[garylinger]

Is this a re-power? Yes, repower. You should be sure you don't have a fuel line restriction. New fuel line from first filter to engine and then from engine back to fuel tank (return line). I changed out an original fuel line and picked up 1000 RPM due to some restriction that had developed. Have you checked your filters? Have not recently checked the one filter (filter on engine is new).

An important clue is the color of the exhaust smoke and how it changes as you approach the maximum RPM the engine will develop with the boat underway. I did not see any exhaust of any color but did not hang over stern to look at thru hull.

 

[kloudie1]

The transmission gear ratio of the new Beta may be very different from the old transmission ratio.. ?? Should be on a plate on the outside of the transmission case. Propeller may be turning faster at the same engine speed of the old rig.. Which would make the prop pitch too big. Check the ratios for a difference Sounds like the prop will have to be adjusted .. But first check Roger's advice on the fuel delivery restriction.

 

[sailingdog]

It does sound like you're overpropped. It could be due to the transmission gearing being different or the fact that the new engine has a different WOT recommendation. I'd recommend trying to drop the pitch about 3 inches...since each inch of pitch raises/lowers the WOT RPM by about 200 revs.

Working with a good prop shop will help a lot. However, I don't think they can modify your existing prop that much.

 

[BobM]

Wow...that 13 inch prop doesn't seem right for your boat...I have the same hull and the same engine as your old one and I have a two blade 16 inch right hand with a 9 pitch.

 

[Maine Sail]

You should ideally be able to hit max rated RPM at max throttle with a clean bottom and prop in smooth current free water. 2900 from 3600 is quite low for a max RPM. You should be turning about 2900 for your cruise RPM, not max.

Universal & Westebeke are very picky on proper RPM. In the spec sheet for all their new engines they will have a statement like the one below. The Universal engines use Kubota blocks just as Beta does. I'd call Beta and see how close they want it..

From the M-25 XPB Spec Sheet:

"Universal recommends a propeller that will allow the engine to turn 3000 RPM underway at full throttle"

3000 on that engine is max rated RPM.

By doing this you also throw your HP curve off. At wide open, now, you are only developing about 13.5 hp not the 16 you paid for..

 

[woodster]

contact michigan prop and inquire about a prop size for your set up they will get back to you usually with in 24 hrs and tell you what size prop you sould be using ...go to there web site and find the prop request sheet and fill out all the info requests and e mail it to them they will reply ....i did that and was told that i needed a 15/10 two blade for my 92-A and it may be 1 in off +or- on the pitch and that can be adjusted with out damage to the prop ....hope this helps

regards

woody

 

[Warren Milberg]

I guess I have a different opinion. I don't know the age of your boat, or the number of engine hours on the engine, but my sense of things is that you have a pretty good situation for a used boat. That is, you can do 7 kts @ 2900 rpms. You aren't going to be able to go much faster than that no matter what you do. Most people I know do not cruise at max speed or max rpms, but at somewhat lower levels (i.e., 60-80% of max). So if you can cruise at say 6 kts at lower rpms, and the engine is not struggling, and no unusual noises/exhaust are evident, I can't see any issues or problems here.

 

[Stu Jackson]

STOP for a minute

New engine, but I'll bet same old tachometer at the cockpit panel. Don't know if you purchased a new alternator with your new engine and connected it to the old tach.

It could well be that EVERYTHING is just fine, but your tach is out of whack with the new engine.

Before you go worrying about new props and new transmissions.

I agree with Warren. I also agree with the other advice, but before you go ballistic with stuff to angst about, think about it first.

 

[Don S/V ILLusion]

Stu is correct ( as usual). All too often, people make rash judgments about engine rpm and prop size and I just read a similar scenario on another forum where it turns out his tach was incorrect. ALWAYS check the easy stuff first.

It is possible you have the wrong prop, however, it only makes sense to first check to make sure your rpm is what you think it is before going to the most difficult and expensive option.

 

[druid]

OK, I've been studying this kind of thing for quite a while. First, Warren's right: check the tach! Second, Roger's wrong (a rarety!) - EVERY sailboat I've been on has been overpropped - it's VERY common (look how good my engine is: I can get 6 knots at only 2000 rpm!). And third, you're not that far off.

I don't think it's lack of power if you're making 7 knots, so don't worry about fuel, exhaust, etc. BTW: I think your hull-speed calc is wrong - a 30-ft (25ft LWL?) would be about 7 knots. And one thing I've learned: all the prop-calcs you can find are only GUIDES - the best way to get the right prop is what you did: measure RPM at WOT. Now, you're getting 2900, and if I know that engine, I think they say 3400 is max continuous. So you're off by 500 rpm. Rule of Thumb is about 400 rpm per inch of pitch gained or lost, so theoretically 1 inch less pitch would get you about 3300. 1.5 would get you about 3500, which would be about right.

I'd recommend (after ensuring your RPM reading is correct!) taking 1.5 inches of pitch out of the prop. Or go to a 14 inch 3-blade Campbell Sailor, but then you'd have a whole new set of pitch problems to deal with.

druid

 

[Roger Long]

Roger's wrong (a rarety!) - EVERY sailboat I've been on has been overpropped

What I meant was that most sailboats have too much engine for the prop because horsepower is comforting to potential customers from a marketing standpoint. Saving money on shaft and prop doesn't impact sales much. The result is many boats not having room to swing the prop that would be a good match for the engine. Most do have a large enough prop to push the boat well in the conditions sailboats normally encounter but, for many, not large enough justify the amount of iron that is being carried around.

Usually, you can drop the engine size and horsepower on a re-power end up with about the same performance which is why my first guess is not that the OP's new engine isn't up to the job.

There is a rational for prop engine mismatch on a sailboat though. The smaller prop has less drag under sail. The amount of time the prop is being over driven, possibly with cavitation, and with higher vibration and fuel bills will be minimal and probably in conditions where that is the least of your worries. Doing the same thing in a powerboat that might spend 80% of its time in that power band has a much bigger downside.

 

[woodster]

haveing spoke with gary in a e mail ...he originally had no tach..so i am assuming he got one with his new engine from bata ......he also went up 2 hp from the original 2qm15 (14hp)....the new one is 16 hp......i am wondering about his prop size myself ...because i had to find one for my boat ...and in doing the research ended up contacting michigan prop for an evaluation ...and they came back with the recomended prop for my S2 9.A of 15/10 at 3000 rpms( two blade) ...and based on the calc...it will run 6.5 knots under power in normal waters....obtw i found one in good condition 15/8 for $20.00...and another one 15/10 for $39.95 ...so i have the best of both worlds...whitch ever one works the best i will have a spare just incase....lol now to find a sail for the furler and be so lucky to fine a good price on it as well ....paticences..... woody..... paticences

regards

woody

 

[woodster]

What I meant was that most sailboats have too much engine for the prop because horsepower is comforting to potential customers from a marketing standpoint. Saving money on shaft and prop doesn't impact sales much. The result is many boats not having room to swing the prop that would be a good match for the engine. Most do have a large enough prop to push the boat well in the conditions sailboats normally encounter but, for many, not large enough justify the amount of iron that is being carried around.

Usually, you can drop the engine size and horsepower on a re-power end up with about the same performance which is why my first guess is not that the OP's new engine isn't up to the job.

There is a rational for prop engine mismatch on a sailboat though. The smaller prop has less drag under sail. The amount of time the prop is being over driven, possibly with cavitation, and with higher vibration and fuel bills will be minimal and probably in conditions where that is the least of your worries. Doing the same thing in a powerboat that might spend 80% of its time in that power band has a much bigger downside.

based on that would you be good enough to tell me if there is a diff in a 2 blade 15/10 and a three blade 15/10 other than the extra blade ......

regards

woody

 

[garylinger]

First, thanks to all for their comments/opinions/advice. It is a brand spankin new engine with a new electric panel including tach. I did not have a tach on the old Yanmar so don't know what I was getting for r.p.m.s but don't remember getting above about 5 or 5.5 knots with it. I like to have my equipment perform as it is supposed to but having said that, I can live with a few less r.p.m.s with that hull speed as long as I am not doing any damage to the engine and as long as there isn't a problem with the engine. I was assuming that it came down to the prop since it will turn over 3600 in neutral. I'm waiting for comments/advice from the Beta rep (Stanley) but wanted to check here for the expertise!

 

[Maine Sail]

Rule of Thumb is about 400 rpm per inch of pitch gained or lost, so theoretically 1 inch less pitch would get you about 3300. 1.5 would get you about 3500, which would be about right.



druid

Druid 1" to 400 RPM is not the rule of thumb for sailboat props. The generally accepted rule of thumb is 1" to 200 RPM but then even that is rarely spot on..

I would disagree with the assumption that "all boats are over propped so it's okay" thinking. I just went through this on my own boat last year and was 300+ RPM off. Westerbeke engineers were fairly adamant that I really needed to be +/- 50-100 RPM max. I am now within about 50 RPM +/- just as my boat was before I re-propped it.

Also when I worked with a surveyor we always tested max rated RPM on sea trials and yes many of the boats were over propped but these were also the boats whose engines were often in rough shape loaded with a sooty transom etc.

Based on Gary's engine, which is a 16 hp rated engine @3600, he is only at 13 +/- Hp at WOT not the 16 he should develop. If he now backs that off to 80% of max throttle he is barely at 10 HP. This could be lugging his engine and could lead to a plugged exhaust elbow or worse.


This is a direct quote from Beta US.

When operating under WOT (wide open throttle) your engine should operate within 200 RPM of the manufacturers specification.

Beta is a little more liberal than Universal/Westerbeke on this but still a lot closer than 700 RPM to max rated is what they want to see.

So for Gary he should be able to turn at a minimum 3400 with 3600 being max rated.

As others have said you'll want to first confirm your tach is accurate with a digital photo electric tachometer as they can often be off. They are quite reasonably priced these days compared to when I bought mine.

 

[Stu Jackson]

Tach & rpms

In addition to Maine Sail's comments, please note that the issue of the tach remains important. While the tach is new to you, how do you know it is accurate? As you most likely know, it is working off the alternator. You might want to consider checking the actual engine rpms off the flywheel compared to what your tach is telling you, again before you go any further.

FWIW, our C34s over the years, since our original 1986 model to the last 2009 models had the same prop, same underbody. The engines went from 21 HP to 35 HP. All were Universal engines.

 

[Dan Johnson]

I'm joining the "Check the Tach" vote...

buy or borrow a laser tach and shoot a reflector on the engine pulley to find out what the engine's RPM is in relation to the tach reading. Without knowing the true engine RPM, all possibilities mentioned are conjecture (but good, valid points none-the-less). If the laser vs. boat tach are within 10% of each other, then prop size or fuel restriction under load may be the answer.

Last year I posted that I found a couple hundred RPM difference between the laser reading and the Yanmar display. Others confirmed that they found differences as well.

 

[druid]

Druid 1" to 400 RPM is not the rule of thumb for sailboat props. The generally accepted rule of thumb is 1" to 200 RPM but then even that is rarely spot on.

Depends on who you talk to! taking an inch out of a 17-pitch powerboat prop is WAY different than taking an inch out of an 8-pitch sailboat prop! As you pointed out, there's more HP at higher RPM, so there's a "positive feedback" thing going on that adds to the rpms.

Also Gary is 700 RPM off not 500RPM..

First, most engines (like mine) that talk of 3600rpm "max" are 3400 continuous. Even the website you quote say it should be "within 200 rpm", so 3400 is fine.

I would disagree with the assumption that "all boats are over propped so it's okay" thinking.

First, it was not my intention to say that just because most boats are overpropped that "that's ok".

Second, yes, I have heard from many dealers who say you MUST be within a few RPM of max, although the guy I bought my Universal from (who'd forgotten more about Atomics and Universals than I'll ever know!) said a few hundred RPM low is fine, as long as you're not smoking or overheating. And I ran my Universal (Kubuta, same as a Beta) 500 rpm low for over 10 years with no ill effects.

As you infer about sooty transoms, the Clue is whether or not it's smoking or overheating. If not, not a big concern (although as you point out he's not getting the most out of his engine)

Bottom line: all the theory in the world is just that: Theory. The ONLY way to ensure the correct prop is to put it on and test it!

And Woody: a three-blade has less slip than a two-blade (also higher-diameter props have less slip than smaller ones). This is the "magic number" about prop calcs: how much does it slip? Slip is basically the difference between how fast you actually go through the water and the theoretical speed you would go based on prop pitch and RPM (Pitch is how far it goes through the water in one revolution, theoretically). Bigger prop or more blades or more efficient blades mean more "grip" and less slip.

Make sense?
druid