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Proper Prop for a Dauntless 15 ?
|Author||Topic: Proper Prop for a Dauntless 15 ?|
posted 03-06-2004 10:48 PM ET (US)
I just bought a 1996 Dauntless 15 with an Evinrude 70 on it.
It jumps up on plane quickly but the top-end is lacking .
It's doing about 5000 RPM at 30 mph. It's got a 17 pitch
prop on it. Should I go to a 19 ?
What kind of performance should I expect out of this hull/motor combination ?
posted 03-07-2004 02:31 PM ET (US)
Congratulations on your new 15' Whaler there in Pace, Florida!
What's the max rpm range on that motor? What's its gearing?
Replacing the 17" with a 19" will lower the rpms. The end result may be even lower boat speed, or a boat that's no faster with less acceleration.
How are you measuring boat speed? Average of GPS readings of two runs in opposite directions to cancel out the effects of current and wind?
How heavily is the boat loaded when you're registering those speeds?
Is the motor trimmed up for maximum rpm when you're measuring max speeds?
Is the motor mounted with the AV plate at or preferably above the bottom of the transom?
That boat's only slightly smaller and lighter than a 150 Sport (15'1" vs 15'5" LOA, 6'4" vs 6'6" beam, and 850lbs vs 900 lbs), yet seems to have more of a V at the stern with the 40% deeper draft (9.75" vs 7"), which would require relatively more power because of the reduced lift. A 150 Sport does about 35 mph with only one fuel tank and pilot, with a 60HP motor. The 70 vs 60HP may or may not offset that.
Hopefully, a 15D owner will come along and add their experience.
posted 03-07-2004 08:06 PM ET (US)
Thanks Moe. I really don't know the answers to your questions. I have only been out in the boat once and that was for a test run. I pick it up tomorrow.
I was measuring the speed by the speedo on the boat. I didn't have a GPS with me.
Hole shot is really quick. I just figured a higher pitch might give me a higher top speed.
Yesterday, the boat was registering about 5500 RPMs and
doing 32 mph. That was with 3 people aboard, total weight
about 600 lbs.
The motor was not trimmed up for maximum speed.
I figured the Dauntless would be faster than the 150 Sport
because of the V hull.
I think I'll repost this in the Performance section which is probably where I should have put it in the first place.
posted 03-08-2004 12:59 AM ET (US)
Don't trust the speedo to be very accurate. A general rule of thumb is to prop it to reach the upper end of the rpm range with an average load. Make sure you trim out when finding the max rpm's and use a handheld gps to find top speed.
posted 03-08-2004 09:25 AM ET (US)
Perry, what should the upper end RPMs be with that engine ?
posted 03-08-2004 02:08 PM ET (US)
Buckda has a '95 Evinrude 70 and previously reported its rpm range is 5,000-5,800 rpm.
I don't see where he reported the gearing, but you can get that by putting a socket wrench on the flywheel nut and counting how many turns it takes to make the prop turn one revolution. For example, my gearing is 2.33:1 and it takes two turns plus 120 degrees (1/3 or .33) to turn the prop one revolution.
Before deciding on a prop change, you need to get the boat out with you alone, if you'll ever be operating it that way, and see how high of an RPM you can get by trimming the motor up. From your report of 5500 with three people and not optimal trim, you may find that the motor overspeeds (i.e. more than 5800 rpm) with just yourself.
The rule of thumb is a 400 rpm drop for a 2" increase in pitch, if prop brand and model remain the same.
Let's say you could've reached 5600 properly trimmed with three aboard. The 19" prop should take that down to 5200. And if we say the properly trimmed motor with you aboard gives 5900 rpm with the 19", the 17" should bring that down to 5500.
As far as real-world speed, Perry's right. Get a GPS and find the average of two WOT, properly trimmed runs in opposite directions.
A V-bottom boat usually takes more HP to go the same speed as a flat-bottom boat of the same weight. If you wind up faster than a 150 Sport, I believe it will be due to the slightly narrower beam, the slightly lighter weight, and the 70 vs 60 HP, in spite of the deeper V.
posted 03-08-2004 05:34 PM ET (US)
So, if it is expected that my boat would be slower than a 150 sport, I wonder how JohnJ80 is getting 42mph at 5000
rpms out of his 15' Dauntless with a 75 Evinrude FICHT and a 15" Stiletto prop ?
That's what he reported when I listed this same question under the performance section. But then there is an identical boat and motor to mine on the Traderonline and the ad says "Top speed 32 mph".
Heck, I don't know if this is the performance I should expect or not.
posted 03-08-2004 05:58 PM ET (US)
I wasn't saying for sure that it would be slower or faster than a 150 Sport. I was just pointing out the places where it has advantages, and where it has disadvantages, with regard to speed. 70HP is almost 17% greater than 60HP, and that's combined with slightly narrower beam and slightly lighter weight. Whether the loss due to the deep-V is less than, equal to, or greater than those advantages, I can't say.
posted 03-10-2004 04:39 PM ET (US)
Thanks for the heads up on this thread Moe -
You should be getting between 5500 and 5800 RPM at Wide Open Throttle for that motor. I'll check on the gear ratio tonight - I have a shop manual for the engine and am sure it includes the specs.
I'm running a 19" pitch prop on the motor and hit about 5600 RPM at WOT fully trimmed.
It gives me about 43MPH on the GPS dead flat calm. I borrowed a 13.75" by 19" prop last summer and saw 5800 RPM and about 40.5 MPH (rough estimates based on memory for RPMs). Bought the boat with a 15" pitch prop (Used by owner to tow daughters on skis) which allowed the motor to bury the tach (6,000 RPM), so I switched it...top speed without over-revving was about 33 MPH - but this prop was somewhat damaged over the course of its use (nicked and chipped).
For the record, I believe my engine is a 1994 model (I may have previously misrepresented it as a '95 - if so, it was a typographical error).
The boat is a 1989 15' Sport GLS (with the fiberglass insert) and top speed was with about 12 gallons of fuel, 180 pound captain, and 1 gallon of oil (plus battery and 12 pound anchor).
I would think that the 15' Classic sport would be faster due to light weight and shallower V....but am happy to race anyone to find out for sure. :)
One thing not yet mentioned that you can check right in the driveway...lower the motor all the way down. Your anti-ventilation plate should be 1/2" - 1" above the bottom of the transom. If your motor is lower than this, the increased drag may be inhibiting your performance.
Please also let us know if you did a compression check on the motor when you bought it (not trying to raise any alarms here...if you didn't..hold on until we eliminate more simple factors).
The compression should come back something like 130 130 130 or very close in variation (I think 10% is acceptable between any two numbers, for insance 132, 130, 129 should be fine).
Let us know what you know, and how it is set up - in the meantime, I'll check out the manual tonight when I get home.
posted 03-10-2004 05:00 PM ET (US)
Also of help:
This will help you later if you ever need to identify broken/missing parts, eh?
posted 03-10-2004 06:03 PM ET (US)
Buckda, I did not do a compression test. I took the boat out and it ran good so I figured it was OK. I mean, if it wasn't getting good compression, it wouldn't spin the prop at 5500 rpm would it ?
The engine is a 1996 and it's on a 1996 Dauntless 15 which weighs about 400 lbs more than your boat. It's got a 17"
pitch prop. So apparently I am getting the proper RPMs out of it. I just didn't want to overrev the engine. Will it hurt it to run at, say, 5300 rpms ? What rpms should I not
consistently exceed when running ?
I have only had the boat out for a test run but best I can remember, it was doing about 33 mph (by speedo) at 5400 rpms. In that range anyway. I guess I need to test by GPS.
posted 03-10-2004 06:05 PM ET (US)
I just realized I previously reported the motor was an Evinrude 70. It's actually a Johnson 70. (like it makes a difference)
posted 03-10-2004 06:54 PM ET (US)
Makes no difference...my shop manual covers both.
By the way - I just had my powerhead rebuilt. Compression was (something like) 130, 128, 72.
The #3 cylinder was scored. I was still cranking out 5,800 RPM and "sounded" just fine at throttle...it had a stutter at idle. I'm not saying there is anything wrong with your motor...I was just asking.
When they finally let me out of here, I'll check on the gear ratio. Standby.
posted 03-10-2004 08:36 PM ET (US)
According to Clymer's Manual, the gear ratio is 2.42:1 (good on 1995-1998 models, 50-70 HP, 3 cylinder.)
Anything else? :)
posted 03-10-2004 09:49 PM ET (US)
Thanks, Dave! You wrote:
I'm running a 19" pitch prop on the motor and hit about 5600 RPM at WOT fully trimmed. It gives me about 43MPH on the GPS dead flat calm. [that's -3.28% slip w/17"]
I borrowed a 13.75" by 19" prop last summer and saw 5800 RPM and about 40.5 MPH (rough estimates based on memory for RPMs). [that's 6.08% slip w/17"]
IIRC, you're running an aluminum Michigan Wheel 13" X 19"P now? Considering its smaller diameter (which I'd think increases slip and raises rpm), and its lower rpm and higher mph, when compared to the prop you borrowed, it seems your current prop is cupped (the negative slip is a clue also) and the one you borrowed wasn't. Using 20.5" pitch gives 3.83% slip at 5600 and 43.2 mph, so it seems to be cupped quite a bit. Would you say thats the case?
posted 03-11-2004 09:38 AM ET (US)
Let me say right off that I am not a mechanic and don't understand a lot of the mechanical stuff with motors.
But, if your cylinder was scored but still cranking out
5800 rpms, what's the problem ? It's still doing what it's
supposed to , right ? As long as you are getting the rpms,
it's going as fast as it can. Or am I missing something ?
I need to get out in my boat and get the exact statistics,rpms/mph and report back. I'll check by GPS.
It's only supposed to get to 70 this weekend so I'll have to wait till it warms up. Just kidding, figured I would get in a jab at our Northern friends. I've got things to do this weekend so it will be probably be the following weekend before I can get out in the boat.
posted 03-11-2004 10:31 AM ET (US)
Using 20.5" pitch gives 3.83% slip at 5600 and 43.2 mph, so it seems to be cupped quite a bit. Would you say thats the case?
Oops... dyslexia strikes again. You reported that speed to be 42.3 not 43.2 mph, so using 20.5" pitch, slip is 5.84 at 5600 and 42.3 mph. And using 20", it's 3.48% at those speeds.
posted 03-11-2004 10:41 AM ET (US)
Florida15, the problem with a weak cylinder is that while the engine may still be statically balanced, it is no longer balanced when running. The cranking pressure gives you an indication of what the combustion pressure will be. Instead of evenly spaced, evenly strong power pulses, Dave had strong, strong, weak, strong, strong, weak, etc. This can set up an internal vibration that's hard on the engine components.
Just because an engine can turn a certain rpm doesn't mean it's in good shape.
posted 03-11-2004 11:18 AM ET (US)
Moe, would it not run rough and sound not very smooth ?
I don't think that's a problem with my engine but I have been thinking about buying a compression checker. Where is the best place to get one and what does it involve ? Are there different types or just one type ?
posted 03-11-2004 10:54 PM ET (US)
Whoa Moe, you just went right over my head!
The 15" pitch prop I have no information about (and am guessing at the pitch based on the serial number that was on it). It has been chewed over the years by the previous owner, so that probably has affected the slip some as well.
The new prop was a Michigan Wheel aluminum (Vortex) 13x19. (Just went out back and checked the box it came in). (that's the one that gives me top speed, fully trimmed). that prop came right out of the box.
The other prop 13.75 X 19 was a loaner from DJHancke (who runs it as a spare on his 18 Ventura). I believe that was a comp prop.
At speed, you really couldn't hear a difference with the motor...only at slower speeds.
I guess what we really need to figure out now is how high your motor is mounted (discernable by checking the anti-ventilation plate in relation to the bottom of your transom).
The concern, as Moe stated, is that if you go to a 19" pitch and don't change anything else, you're going to get fewer RPMs that 5,000. And as Sal likes to say, think of a boat motor like running your truck up the side of a mountain at a steep angle...now think about upshifting to 4th gear and letting it stutter up at 1,000 RPMs...she isn't going to last long...it's very important to get your engine in the recommended RPM range at WOT. In your case, this is about 700 or so RPMs above where you're currently at.
As for checking compression...it's probably best to just have a mechanic take a look-see. Since you've just bought the boat/motor, he can hook it up to the diagnostics and give it a general checkup and let you know what, if anything, is going on.
It's a small price to pay (will cost you like 30 bucks for 1/2 hour of his time) for greater understanding of where you're at with the new motor.
posted 03-11-2004 10:58 PM ET (US)
By the way, Moe -
The prop is cupped... see: http://www.michiganwheel.com/MIWheel/products/outboard/vortex_main.htm#vort
posted 03-12-2004 09:12 AM ET (US)
Thanks, Dave. FWIW, I've got a fair amount of experience on motorcycle chassis dynos, not only the acceleration drum type, but also those with an eddy current brake for tuning part-throttle performance. I don't agree that the motor always has to be propped to the high side of the max rpm range at WOT or you're hurting it. One of these days I'll write something up on it... complete with illustrations. :)
posted 03-12-2004 09:46 AM ET (US)
Would love to see that article and the debate that will surely ensue.
Looking forward to it, and, as a guy who doesn't have a ton of mechanical knowledge, I defer to those with greater knowledge regarding the max RPM range idea, although the majority opinion seems to lean in the favor of that camp.
posted 03-12-2004 10:50 AM ET (US)
Buck, I'm not sure how high my engine will go. The guy I bought it from seem to think you shouldn't go above 5300 rpms for long so I didn't. I still had throttle left at 5300 but was afraid to go higher since he warned me not to go too high.
If I can go up into the 5800 range for WOT, I really don't think I have a problem at all. Based on the throttle I had
left, I really think it will get up into that range. I just need to get out and check it.
So, the question is : Just what rpm range is it safe to run in for any length of time ?
posted 03-12-2004 01:51 PM ET (US)
Your motor will last a lot longer if you keep most of your long runs under 70-75% of max rpm. Short runs of 80-90% won't make a big difference in wear and an occasional blast at WOT is okay too. Constantly running 80-100% will see you doing a top end job a lot sooner than if you'd taken it easier.
posted 03-12-2004 05:45 PM ET (US)
Hmm...well, I *think* we were operating on the assumption that you were telling us what speed you were getting at WOT.
About 30 MPH at 5,000 RPM sounds about right given that you weren't at WOT.
You should be propped so that you CAN'T push it past 6000 (allowing the engine to over-rev)...that will damage the engine...at WOT, you should be around 5,600 - 5,800 RPM to be comfortable.
It's true that you shouldn't consistently run at WOT, nor do it for long, but when you're out having fun, the occasional WOT blast of speed could actually be good for you! (Gets the blood moving).
There is a lot of power in those last 800 RPMS, which should get your speed right around 40 MPH with the motor fully trimmed out.
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