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Author Topic:   Prop for 70 HP on a Dauntless 15
rkong posted 02-05-2003 01:17 AM ET (US)   Profile for rkong   Send Email to rkong  
I currently have a 14x17 prop on my D15 70HP Johnson and am unsatisfied with the performance. With 2 people (300lbs), 15 gallons of fuel, 1 battery, and some fishing gear, I get about 30-31 at WOT on the GPS. Tach is at ~5250. I think I should be able to do mid to high 30s at WOT. I also have a doel fin. Any recommendations for a prop?
gschrimp posted 05-01-2003 12:44 AM ET (US)     Profile for gschrimp  Send Email to gschrimp     
Sounds like you have some other problem.

For a comparison I have a Yamaha P75 on my 1996, 15' Dauntless. Running at 5000 RPM with full fuel (15 gallons), 2 POB (370 #'s), two full coolers and lots of other gear I can just squeak out 40mph on the GPS with a 13x19 SST at optimum trim, but it does chine walk a bit at that speed.

Running a 4 blade SS 13x13 (used mostly for water skiing) with 2 POB and running at around 4450 RPM towing a skier, top speed is slightly under 33mph.

I do have a full SS plate attached to the motor for quicker hole shots and better handling. I don't recall the manufacturer name though.

Below is a URL for the people who helped me find the right prop for skiing. They are the best!

When I needed a ski prop they took their best guess and sent me a 4-blade Powertech 13x15 with a guaranteee that if it didn't work they would keep working with me until I found the proper one. I received the prop a couple of days later from the manufacturer and tested it. Though it was an improvement it didn't feel right. They then shipped me out a 13x13 and told me to send the one I had back. Two days later I got the 13x13 and it worked perfect. I sent the other one back and was only paid for the UPS.

They spent close to 2 hours with me on the phone. This is the best customer service I have ever received in over 35 years of boating and I once helped out at a local marina.

Although the prop itself was around $350.00, really not much more than any other performance SS prop, I saved a bundle by getting the correct prop that worked best on my boat. It was well worth the money and time spent.

I suggest you give them a call and they will work with you.

Sal DiMercurio posted 05-01-2003 06:51 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Kong, here we go again with a 14 diaqmeter prop on a 70 hp engine on a small boat.
Whoever rigged that boat really didn't care or know how it would perform.
Absolutly no reason for a v6 prop [ 14" diameter ] to be on a 3 cylinder 70 hp engine.
I'v watched dealers do just what you have on your boat, & to add insult to injury, they mount the engine all the way down on the first holes,....why?....I guess becuase they really don't know how to fine tune a setup.
Kong, your boat should be in the low 40s, running a 12 or 13 inch diameter with a 19 pitch.
You can get a s/s performance prop for that engine & boat from aftermarket for about $239.00
A Stiletto will make that boat dance.
One of your problems could be the DoelFin, as it forces the bow down & the "WHOLE" boat sticks to the water, not being able to ride on the last 1 ft of boat as it should.
You need air under it to perform.
Also, raise that engine up to the 3rd set of holes.
Do "NOT" buy a prop for that engine & boat setup thats bigger then 13 diameter.
If you plan on sking with it, you will need to drop your pitch by 4 inches to a 15, but watch your tach so she "dosen't" over rev to 6,500
I think the recommended highest rpms for that engine is around 5,600.....don't let her run above it.
Dick posted 05-01-2003 08:39 PM ET (US)     Profile for Dick  Send Email to Dick     

The test data sheet that I have from Whaler shows the 15DA with a 70 Johnson toped out at 39.75 mph with a 13 1/4 X 17 SS prop.
They also tested it with a 12 3/4 X 21 and toped out at 43 mph. I'll send you a copy.


Sal DiMercurio posted 05-01-2003 10:00 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
If you notice the results from the above tests, you don't see any testing with a 14 diameter, just the 13.25 & the 12.75..........why.......because a 14 dia prop dosen't belong on a 70 hp engine.
Did you notice the best results were with a 12-7/5 diameter & 21 pitch.
Diameter gives power to plane, pitch is thrust, which equates to speed.
The less pitch you have in a prop & the larger the diameter, the better the hole shot, but thats about it, because speed needs pitch to push you there & to much diameter will not allow the prop to spin fast enough to give you much speed.
Rule of thumb, always run the smallest diameter possible with the most pitch if you want speed.
On the other hand, if you own a house boat, your gonna need a 14 diameter by about 11 pitch which means you might see 7 mph at wot, yet, that engine has to be able to reach that recommened high rpm range of 5,500 or 5,600 or you will burn it up by lugging it to death.
Sal DiMercurio posted 05-02-2003 09:17 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
If that 70 is a looper, the max rpm is 6,000.
gschrimp posted 05-04-2003 11:16 PM ET (US)     Profile for gschrimp  Send Email to gschrimp     
Here are some more interesting numbers having to do with fuel and props.

Running the 13x19 3-blade SST on the Dauntless the average fuel economy is a tad over 5 mpg. The only time it drops below that is if we are running in exceptionally heavy seas and it has never exceeded 5.6 mpg on long distance cruising on calm waters.

With the 13X13 4-blade fuel efficiency drops down to just a bit over 4 mpg (exception of course is skiing). Best result was a very calm day on Lake Michigan where at cruise (22mph) we averaged 4.7 mpg on a 200 mile round trip over a 2-day period.

Both tests are compiled using a minimum of 1000 miles each as a benchmark.

rkong posted 05-05-2003 02:56 PM ET (US)     Profile for rkong  Send Email to rkong     
Thanks for all the info guys. I gave the 13x19 a try and it significanly lowered my RPMs to ~4500 WOT and I still only got 30MPH on the GPS. I think I'll give the 13.25x17 a try and see how it runs.
Sal DiMercurio posted 05-05-2003 05:51 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Your going to have to go to a 13 x will give you 5,300 rpms & more speed.
If you can find a 13 x will get 5,500 rpms & your engine will be much happier & so will you.
By going to the 19, you lowered your rpms far to much, more pitch only means more speed when the engine can twist it fast enough, obviously it can't.
Your way over propped.
Sal DiMercurio posted 05-05-2003 06:10 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Kong, obviously you didn't take heed to me telling you that dole fin is not going to allow your boat to ride high enough to allow it to gain speed.
Take the fin "OFF".
By taking it off, you will allow the boat to ride as it should, "NOT" flat as a pancake & stuck to the water like glue.
That boat needs to ride at wot on the very last 1/4 of the bottom.
The fin is forcing the boat down on the water & the whole bottom is wet, not the last 1/4 of it.
Take the damn thing off & try it, then come back & tell us how she liked it.
Remember, all the trim in the world isn't going to help if that fin is in the water.
whalersman posted 05-05-2003 08:34 PM ET (US)     Profile for whalersman  Send Email to whalersman     
I agree with Sal... Take Off the Fin and give it a try..

I don't know what year your 70 HP Johnson is, but the majority of 70 HP Evinrude/Johnson's came with a 13 1/4 x 17 prop in the 1980's... For many people this prop worked well but I find that using the optional size prop listed in my manual works great for me. 13 3/4 x 15 gives me a better Hole Shot and more RPM's at WOT... about 5300...

My manual says for the 1985 70 HP Evinrude, that it develops maximum 70 HP at 5000 RPM's. My engine is mounted on a Montauk so I can't exactly inform you about your Dauntless 15 but your RPM's should fall within your Owners manual specifications. The specifications for my RPM's are between 4500 and 5500 RPM at WOT...

Let us know what your outcome is...

rkong posted 05-05-2003 08:36 PM ET (US)     Profile for rkong  Send Email to rkong     
It probably is the fin stuck in the water on plane keeping the boat slow. But I can't take it off permenantly. The boat porpoises way too much without it.

The engine is mounted on the 2nd hole from the top. Perhaps the engine is too low, because the cavitation plate is lower than the keel of the boat, thus keeping the fin in the water. From what I read on the forums, the fin should only slow me down 1-2 mph top end. And I never go that fast. 20-25MPH is fine for me. Its just strange I'm not getting above 30MPH WOT.

I'm going to take off the fin to see how it performs, but theres no way I can leave it off permenantly unless I solve the porpoising problem.

Is there any D15 that doesn't have a problem porposing without a fin?

I don't think I need to go al the way down to 13x15 to get 5500 RPMs. Right now I'm at ~5250 or so with a 14x17, so shouldn't a 13.25 x 17 increase my RPMs a little bit?

Barry posted 05-05-2003 10:04 PM ET (US)     Profile for Barry  Send Email to Barry     
I think your engine is too low. Usually the anti-ventilation plate should be at least even with the bottom of the hull and preferably .5-1.5" above that. The first thing I would try would be raising the engine. Then check your performance and make a decision on the prop.

I would leave the Doel-Fin on. I've heard that the 15-Dauntless can have porpoising problems and the Fin should help with that. It should only slow you down 1-2 mph and should allow you to plane quicker and stay on plane at a lower speed. They are simple enough to remove though if you want to try it but I would only change one thing at a time.

Sal DiMercurio posted 05-06-2003 12:15 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Number 1, your engine is to low, thats why you porpoise like a frog.
Number 2, theres no way you will get the bottom out of the water enough with that fin on there, it just plain pushes the bow to low.
Number 3, the antivent [ cavitation ] plate needs to be at least 3/4" "ABOVE" the bottom to stop the porpoising.
Number 4, You don't need 13 inches of diameter for that boat, a 12.25 will help you get more speed.
Number 5, if that dole fin is below the surface at wot, you will loose a whole lot more then 2 - 3 mph, try 10 mph.
Once you allow some air under that boat, she's going to break loose from that vise grip & move as you want.
If you get rid of the fin, raise the engine as high as it will go & it still leaps like a frog, make yourself a set of plates for the stern & adjust them with a lever if you can't afford the hydrolic ones, this will cure the porpoising & allow you to get air under the boat.
rkong posted 05-20-2003 04:56 PM ET (US)     Profile for rkong  Send Email to rkong     
So here's the latest news. I took the doel fin off and gave it a run. It turns out she doesn't porpoise as much as I thought.

The first time I took her out she had a trolling plate on her and after that I immediately took it off and installed the doel fin. I never tried her without anything on the cavitation plate. I now think it was the trolling plate making her porpoise like a frog.

So trying her today without anything on the cavitation plate was good. Heading into the wind, I didn't notice any porpoising. Heading with the wind she did porpoise a little but that wasn't too big a deal.

As far as speed/RPM, I could get her to 5000RPM with just myself and 15G of gas on board. I'm still running the 14x17 prop. I know its too big. Top speed was 33MPH at 5200RPM after playing with the trim. At 3500 RPM, I'm cruising at 20MPH.

where2 posted 05-21-2003 01:24 PM ET (US)     Profile for where2  Send Email to where2     
Keep reading Sal's comments, and raise the engine so the cavitation plate is atleast level with the bottom of the keel, if not 3/4" of an inch above it....

There's more performance left in that boat...

rkong posted 05-21-2003 04:26 PM ET (US)     Profile for rkong  Send Email to rkong     
Just raised the motor. It now sits on the bottom holes. The cavitation plate is now 1 inch above the keel. Will post results after I give it a test run.
Perry posted 05-21-2003 04:51 PM ET (US)     Profile for Perry  Send Email to Perry     
Keep reading Sal's comments. Get rid of that 14" diameter prop and replace it with a 13 X 17 prop.
Perry posted 05-21-2003 04:54 PM ET (US)     Profile for Perry  Send Email to Perry     
Oops, I meant a 13 X 15 prop.
rkong posted 05-21-2003 07:35 PM ET (US)     Profile for rkong  Send Email to rkong     
I'll try a prop change a try after I see what raising the motor does first.

From the boston whaler specs, they tested this boat with the following results.

70 HP Johnson 41MPH @ 6100 RPM. With a 13.25 x 17 SS Prop, 2 People 340 lbs.

Should I follow what them and use the same pitch/diameter prop? Right now I'm at 5000 RPM with a 14x17. Will dropping the diameter by 3/4" give me another 1000 RPM?

Perry posted 05-21-2003 11:15 PM ET (US)     Profile for Perry  Send Email to Perry     
What year is the motor? When BW did the test, the motor and boat were new. Try raising the motor and see what happens then change props. Going down 3/4" will not give you 1000 more rpm, thats why I suggest a 13.25 X 15 prop.
rkong posted 05-21-2003 11:33 PM ET (US)     Profile for rkong  Send Email to rkong     
The motor is a 96. I forgot to mention that when I tried the 13x19 with 4500 RPM WOT it was cupped. I think that makes a difference and is like a 13x21 uncupped? Right? Its on the 4th set of holes from the top right now. Will give her a whirl tomorrow.
where2 posted 05-22-2003 01:13 PM ET (US)     Profile for where2  Send Email to where2     
Looking forward to the results...
JohnJ80 posted 05-25-2003 06:59 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
I'm in the process of tuning my boat up with a new 75HP Evinrude FICHT. I used to have the 94 version of the same motor you have. It had gotten pretty tired and I wanted to replace it while it still had some value.

I had a tiger shark fin on it. I had porpoising problems before I put it on.

My new motor was mounted all the way down so that the AV plate was about 1/2" below the bottom of the boat.

The boat as set up now, pounded terribly in any chop. She also porpoised like crazy unless the water was dead calm if the trim was raised at all.

I just moved the motor up one hole so that it is now about a 1/2" above the bottom of the boat. Huge Difference! I'm going to take it up another hole and then add a Grand Island Marine fin (Whaler recommendation). I'm going to add the fin first since that is easier and the Grand Island fin doesn't require holes to be drilled.

When I moved it up the I got huge trim range back - about where I was with my 70HP like yours. The pounding was greatly diminished and the porposing was hugely diminished. I think I will get alot more back when I go up another hole. More when I add the fin.

So, the motor vertical position is a big deal. Small differences seem to make a large difference in performance and boat dynamics.

My boat with the old 70HP probably hit low 40's. I now have a speedo on my fishfinder and on a calm day with the new motor I hit 45mps with the trim up pretty far. I am pretty sure I was hitting ~40mph with the old motor.

If you pick the motor up, you are reducing the hydrodynamic drag each time you go up. This is, I think, exponential (amount of drag) with water speed past the lower unit. So, if you keep picking it up until you get to the point where the prop ventilates (or worse, your water intakes are ventilated) you should keep going faster from a drag perspective.

The thing I don't understand is the prop selection, others are better at that than I (see above). I also can't comment on the hull speed max on a D15 but you should be getting much higher top end speed.

I just came off the water after raising it a hole. Pretty choppy on the lake today with all the traffic and a huge improvement noticed. I will update after I get a test in better conditions.

hope that helps.


rkong posted 05-27-2003 07:46 PM ET (US)     Profile for rkong  Send Email to rkong     
Well I'm back from a test run of the boat with the motor mounted all the way up (4th hole from the top), with the cavitation plate 1" above the keel. Unfortunately, I didn't get the results I wanted. I actually porpoise more with the motor higher.

With the motor mounted on the second hole from the top, I noticed minor porpoising (without a fin) that was no big deal. Now on the bottom hole, the boat porpoises all the time. Even with the trim all the way in. I didn't even try to bring her up to WOT because the porpoising was just getting worse the faster I went. This is all without a fin.

Now after adding a fin, I was able to get her to handle better. I could get her to WOT at 5400 RPM with the 14x17 prop. However, she still had the tendency to porpoise at 3500-4500 RPM even with the fin on. This was less than without the fin, but still not to my liking. I'm going to lower the motor down a hole and see how she likes that. One good point is like John says, the boat doesn't pound as much with the motor higher.


Sal DiMercurio posted 05-27-2003 10:39 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Rkong, yes you can go up to high.
Please throw that 14 diameter in the bay & get a 12.50 - 13 or 13.25 x 17 or 15 pitch.
No you absolutly will not gain 1,000 rpms by going down 3/4" in diameter, lucky to get 100 - at most 150 rpms by going down an inch.
JohnJ80 posted 05-28-2003 12:31 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     

Interesting that you are getting very similar results to mine with a lighter motor. That tells me that the motor weight is not really my problem.

I am porpoising between 30-40mph with the worst range being 30-35mph. If I goose it up to WOT, the porpoising goes away. Below 30mph it seems to be much better behaved too. I am placed at the second hole from the bottom leaving the motor about 1/2" (best I can tell while in the water) above the bottom of the transom.

I did notice that when I moved up a hole, the porpoising seemed to come about easier in that range, but it wasn't as pronounced.

Whaler tech support tells me that they recommend 0-3/4" above the bottom of the transom for the cav plate placement. They also told me that this is very conservative and that if you wanted to go higher it is up to you and how you use the boat. They also told me that lots of people do run them higher.

My Grand Island Marine Turbo Lift fin gets here next week. That installs without drilling and is supposed to work well - I have talked to them maybe 5 times about this plus Whaler tells me they get great results with them too. The GI guy is pretty sure it will knock off the porpoising.

My top end speed looks like it improved from about 42 mph to about 49mph by going up one hole. that is measured from the little paddlewheel sensor on the fishfinder, which is probably pretty accurate. I don't care about top end speed, I care about ride and the porpoising.

If the fin and the vertical trim don't fix the problem, then I am going to look into prop selection. Whaler told me that the type of prop and size has a large influence on this too. I don't understand the dynamics of that, but I'll give it a try too. I'm going to get this back to the silky whaler ride I had before.

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