Dauntless 15 Yamaha F70

Optimizing the performance of Boston Whaler boats
adk_bearshark
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Dauntless 15 Yamaha F70

Postby adk_bearshark » Fri Aug 14, 2020 12:35 pm

I just re-powered a Dauntless 15 with a new Yamaha F70. I used the [Turning Point] propeller wizard for a recommendation. It gave me the options for a 14 x 13 or 14 x 11 propeller. I bought the 13-pitch to test first.

14 x 13-pitch test run: [boat speed was] just over 30-MPH and [engine accelerated to] maximum speed at 6300-RPM at full throttle. I feel like [the boat speed with this boat-engine combination] should be at least [35-MPH] with no debate.

Readers: give me a little guidance on which direction to go for propeller [diameter and pitch].

If you have a DAUNTLESS 15 boat with a Yamaha F70 engine, please let me know what propeller you use and the top boat speed obtained.

[Edit -corrected boat and engine model names]

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Phil T
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Re: Dauntless 15 Yamaha F70

Postby Phil T » Fri Aug 14, 2020 2:40 pm

While I do not have a specific boat model/engine prop recommendation for the Dauntless 15/F70, I will use similar models to provide some guidance.

Owners who have the F70 on the classic 15 Sport recommend:
13 x 16 PowerTech SCD
13.25” x 17 PowerTech SCD3
13-1/4” x 17” Turbo 1

Owners with an F70 and a classic 16'7 models Katama, Nauset et.al. recommend:
13.25 x 14 PowerTech SCP
13.25 x 14 Yamaha Performance Series/Turbo 1
13 x 15 Yamaha Painted Stainless Steel
You will see the first number is the diameter and the second is the pitch.

The 15 Sport has a hull weight of 360 lbs
The 1994-1998 Dauntless 15 has a hull weight of 730 lbs.
The 16'7' models have a hull weight of 650-900.

In looking at a universal prop calculator, I enter the engine WOT (6300), gear ratio (6300), prop slip (5%) and target boat speed (38) and it returns a pitch of 15.

With this information, I would suggest the props listed above for the 16'7 models above.

Note the size listed is for the specific make/model of prop and is not interchangeable since prop design/performance is not universal across brands or even models. The engine should be mounted two holes up, top bolt in third hole down from top.
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Re: Dauntless 15 Yamaha F70

Postby biggiefl » Fri Aug 14, 2020 2:53 pm

I agree with Phil, a 14x13 sounds like a prop for a pontoon boat.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

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Re: Dauntless 15 Yamaha F70

Postby adk_bearshark » Fri Aug 14, 2020 3:05 pm

[The engine mounting height] currently [is one-hole up].

Will [a change to engine mounting height two-holes-up cause] much performance difference?

I want all-round performance, not just top end boat speed.

I ordered a turning point 13.25 x 17p aluminum three-blade to test before buying a stainless steel propeller.

I want to use a stainless steel propeller once [the propeller pitch and diameter] are dialed-in better.

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Phil T
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Re: Dauntless 15 prop Yamaha F70

Postby Phil T » Fri Aug 14, 2020 3:23 pm

Engine height and prop selection is not about top speed necessarily. It is about rigging the motor so the engine runs the most efficient and peforms well at all speeds.

An analogy is riding a ten speed bike. You want to select a gear that allows you to go at a good speed, with low effort on the flat and be able to pedal up steep hills too.

The the engine rigged and propped appropriately will allow the boat to reach ~38 mph at WOT, running light, it will also be able to run well, 32-34 mph with 3 passengers and gear.

Testing has shown that most engines will gain or loose from 150-200 rpm's for each bolt hole lowered/raised due to more or less drag of the lower unit in the water.

I am not fluent with the blade design/performance attributes of the Turning Point models to have an opinion on its performance on your boat.

I would suggest you install the new prop, launch the boat with a full fuel load, light gear, running solo and report WOT rpm and GPS speed (note wind and current) and report back.
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adk_bearshark
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Re: Dauntless 15 Yamaha F70

Postby adk_bearshark » Mon Aug 17, 2020 8:49 pm

Results of testing a 13.25 x 17-pitch aluminum [Turning Point] propeller were not good. Boat SOG was 26-MPH measured with GPS receiver and WOT engine speed was 4,100-RPM.

I was unable to measure current or wind speed, but I’m on a lake so current is not much a factor. When I tested the boat today there was minimal wind and the lake was glass.

The specifications says this Yamaha F70 has a WOT range of 5300 to 6300-RPM

Those specified engine speeds have me confused why my results are so much different. I am hesitant to order one of the stainless propellers.

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Re: Dauntless 15 Yamaha F70

Postby biggiefl » Tue Aug 18, 2020 10:19 am

Who installed the engine? It is usually the installers job to test run the boat and make sure everything is adjusted correctly and then make a prop recommendation or possibly supply the correct prop at that time. You are not going to gain 2200 rpm with a prop change, something looks wrong.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

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Re: Dauntless 15 Yamaha F70

Postby Phil T » Tue Aug 18, 2020 12:15 pm

Have you completed the required 10 hour break-in procedure as detailed in the manual? If not, complete the procedure and then check performance.

Please go back and re-read my post.

Prop size/performance is NOT universal across brand/model. A Turning Point aluminum x by 17 will not run the same as a Brand Y, model z of the same size due to different blade geometry.

Also note that after spending ~$9k on the engine, your dealer wants you to spend $125 on the one element that makes the boat actually go. Like spending 40K on a car and then installing big wheel tires.

If you have finished break-in procedure -------

Your aluminum prop has too much pitch which is contributing to the low RPM's. Could also be the model of prop you selected (see recommendations above)

Please provide a few photos of the boat. Does it have a bimini? Bottom paint, gear, etc.

You don't detail the passenger count, fuel load, gear aboard.

What is the engine height on the transom? How many empty holes above the top bolt?

Did you trim out the engine to the point the rpm's stopped increasing?

Details are really important to troubleshooting.
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Re: Dauntless 15 Yamaha F70

Postby jimh » Tue Aug 18, 2020 4:19 pm

Phil T wrote:How many empty holes above the top bolt?


Good method to elicit the mounting height as "number of holes above lowest," which is the preferred method of description of engine mounting height. There is the following correlation:

Engine Mounting Height

Holes above lowest Number of empty holes above top bolt

None = Lowest position
One hole up = one empty hole above top bolt
Two holes up = two empty holes above top bolt
Three holes up = three empty holes above top boat
Four holes up = four empty holes above top boat.

But the best way to describe engine mounting height is to use the preferred method described in the pinned article on ENGINE MOUNTING HEIGHT that is at the top of the listing in this forum, which is to use a description of the number of holes raised above lowest position, where a hole is 0.75-inches.

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Re: Dauntless 15 Yamaha F70

Postby jimh » Tue Aug 18, 2020 4:20 pm

adk_bearshark wrote:I feel like [the boat speed with this boat-engine combination] should be at least [35-MPH] with no debate.


Please explain the basis for this assumption. I don't have any argument with the stated speed. I am just curious how you came to hold this opinion.

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Re: Dauntless 15 Yamaha F70

Postby adk_bearshark » Wed Aug 19, 2020 1:59 pm

jimh wrote:
adk_bearshark wrote:I feel like [the boat speed with this boat-engine combination] should be at least [35-MPH] with no debate.


Please explain the basis for this assumption. I don't have any argument with the stated speed. I am just curious how you cam to hold this opinion.


Just from researching similar boats. I previously had an F40 on this boat and saw 24.5-24.9 running solo and a 10 galling tank. I was hoping almost double the horsepower would give me 10mph on such a small boat.

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Re: Dauntless 15 Yamaha F70

Postby biggiefl » Wed Aug 19, 2020 2:37 pm

Is 24-25 normal with a 40hp?
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

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Re: Dauntless 15 Yamaha F70

Postby adk_bearshark » Wed Aug 19, 2020 3:45 pm

Phil T wrote:Have you completed the required 10 hour break-in procedure as detailed in the manual? If not, complete the procedure and then check performance.

Please go back and re-read my post.

Prop size/performance is NOT universal across brand/model. A Turning Point aluminum x by 17 will not run the same as a Brand Y, model z of the same size due to different blade geometry.

Also note that after spending ~$9k on the engine, your dealer wants you to spend $125 on the one element that makes the boat actually go. Like spending 40K on a car and then installing big wheel tires.

If you have finished break-in procedure -------

Your aluminum prop has too much pitch which is contributing to the low RPM's. Could also be the model of prop you selected (see recommendations above)

Please provide a few photos of the boat. Does it have a bimini? Bottom paint, gear, etc.

You don't detail the passenger count, fuel load, gear aboard.

What is the engine height on the transom? How many empty holes above the top bolt?

Did you trim out the engine to the point the rpm's stopped increasing?

Details are really important to troubleshooting.


Break-in procedure has been completed.

The engine is mounted one hole down from the top hole. 2nd hole. See diagram below

Top Of transom
O
O<— current engine mount hole
O
O
Bottom of transom

I ran the boat solo. I have a bimini top that is laid back flat when under way and bottom paint. 10 gallons of fuel and a pretty much empty rtic cooler. I trimmed up until rpm began to decline. This was the case on all of my test drives.

I previously had an F40 on this boat with Nauticus trim tabs and would do almost 25mp. This was mounted on the very top hole.

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Re: Dauntless 15 Yamaha F70

Postby Phil T » Wed Aug 19, 2020 3:56 pm

Was the engine installed by a dealer or did you do it yourself?

I would ask the dealer to borrow a different prop. Something is very wrong.

A Yamaha dealer should have a Yamaha prop available. Look at the list I posted.

You should be able to hit 40 mph easy.

For the time being, remove the tabs to eliminate them from the equation.
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Re: Dauntless 15 Yamaha F70

Postby adk_bearshark » Wed Aug 19, 2020 4:07 pm

Phil T wrote:Was the engine installed by a dealer or did you do it yourself?

I would ask the dealer to borrow a different prop. Something is very wrong.

A Yamaha dealer should have a Yamaha prop available. Look at the list I posted.

You should be able to hit 40 mph easy.

For the time being, remove the tabs to eliminate them from the equation.


I installed the motor myself. I’m a certified Yamaha Outboard Tech that works at the dealer I purchased the motor from.

I will remove the tabs tonight and try the 13.25x17p that I have and report my results. I should have mentioned that in my original post. Maybe too much stern rise?

My dealer doesn’t usually sell crate motors or anything below 115hp Yamaha, so I know I don’t have any propellers from your list in my parts room. The closest I had was a 13.5x15p prop that I tested prior to ordering the 13.25x17p. I will have to order one of the Yamaha propellers from your list.

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Re: Dauntless 15 Yamaha F70

Postby jimh » Thu Aug 20, 2020 8:54 am

Ignoring any weight increase, going to 70-HP from 40-HP should increase the speed of a moderate V-hull boat by a factor of (7/4)^0.5 or 1.3-times.

If the boat speed with 40-HP was 25-MPH then the expected speed increase would be to 25 x 1.3 = 33-MPH with 70-HP.

The 70-HP engine very likely weighs more than the 40-HP engine, so the new top speed will be slightly lower than 33-MPH.

Maybe this boat is unusually heavy.

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Re: Dauntless 15 Yamaha F70

Postby adk_bearshark » Thu Aug 20, 2020 11:47 am

jimh wrote:Ignoring any weight increase, going to 70-HP from 40-HP should increase the speed of a moderate V-hull boat by a factor of (7/4)^0.5 or 1.3-times.

If the boat speed with 40-HP was 25-MPH then the expected speed increase would be to 25 x 1.3 = 33-MPH with 70-HP.

The 70-HP engine very likely weighs more than the 40-HP engine, so the new top speed will be slightly lower than 33-MPH.

Maybe this boat is unusually heavy.


That is very unfortunate to hear. I was expecting much better results.

I just moved my motor up one hole.
Top of transom
O
O
O<—top bolt, new mounting position
O
Bottom of transom

Going to test shortly and report back.

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Re: Dauntless 15 Yamaha F70

Postby jimh » Fri Aug 21, 2020 9:11 am

If you describe the engine mounting height in a relative comparison, for example by saying the engine was raised by one hole, we don’t know what the mounting height of the engine will be unless we already knew its prior mounting height. That information might be given many posts earlier, and finding it can be difficult in a long thread.

A much clearer and much more direct method to inform readers of a new mounting height is just to say, for example, the engine was changed to two-holes-up mounting height from one-hole-up. This way readers know the new and old engine mounting height in one sentence.

Using a text diagram to illustrate the mounting height is not really necessary, as the readers—at least the readers likely to be able to offer informed advice—know already what a particular engine mounting height will look like in terms of where the empty holes will be in relation to the hole with the bolt.

Every discussion of engine mounting height should not have to evoke a tutorial on how to describe engine mounting height. The pinned article at the top of this forum lays out the preferred method, which is clear and unambiguous.

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Re: Dauntless 15 Yamaha F70

Postby jimh » Fri Aug 21, 2020 9:20 am

adk_bearshark wrote:
jimh wrote:If the boat speed with 40-HP was 25-MPH then the expected speed increase would be to 25 x 1.3 = 33-MPH with 70-HP.
That is very unfortunate to hear. I was expecting much better results.

The speed estimate I gave is according to naval architect George Crouch. It is discussed in “The Propeller Handbook” by Dave Gerr. It has been shown in many actual examples to be a very good method of speed estimation for planing hull boats.

Going faster always takes more horsepower than you’d think.