1964 NAUSET 16 Yamaha F70A

[Dave_a]

I am now getting a new Yamaha F70 four-stroke-power-cycle engine rigged on my 1964 NAUSET 16.

Give me advice on propeller pitch and propeller diameter.

The hull weight is 800-lbs. A 12-gallon capacity fuel tank is mounted under the pilot seat. A battery is mounted under the console. Typically two adults will be aboard.

[jimh]

A few more facts are needed.

Q1: what engine mounting height have you told the dealer to install the new engine on the NAUSET 16?

Q2: where are the existing engine mounting holes drilled in the transom?

Q3: what is the gear ratio of the Yamaha F70 engine?

Q4; what is the maximum recommended engine speed at full throttle for the Yamaha F70 engine?

Q5: what is the total weight of the boat when fully rigged and in the water?

[jimh]

Give me advice on...propeller diameter.

The diameter of a propeller must not be larger than the propeller aperture on the engine gear case. This raises a new question:

Q6: what is the maximum propeller diameter possible in the propeller aperture of the Yamaha F70 engine?

Generally the diameter of propellers is not a parameter that can be selected separately from the propeller pitch. A propeller maker creates propellers of various combinations of pitch and diameter as are appropriate for the application to the horsepower range of outboard engines those propellers will be used with.

The propeller diameter is also related to the propeller hub diameter. The propeller hub diameter must match the engine gear case. This also raises a new questions:

Q7: what is the hub diameter of the gear case on the Yamaha F70 engine?

[Dave_a]

I didn't know there was going to be a quiz, Jim. I was hoping a forum member with a similar engine and hull could chime in on what prop he had on his vessel. Prop selection at a yamaha dealer is often relegated to what the dealer has in stock, not necessarily what is optimum.
To wit:
Q1: what engine mounting height have you told the dealer to install the new engine on the NAUSET 16?
Answer: cavitation plate even with hull bottom minimum.

Q2: where are the existing engine mounting holes drilled in the transom?
Answer: top holes normal location as per Yam. template. Bottom holes are raised to exit into splash well as per BW Bulletin 10-84

Q3: what is the gear ratio of the Yamaha F70 engine?
Answer: 2.33

Q4; what is the maximum recommended engine speed at full throttle for the Yamaha F70 engine?
Answer: 6300rpm

Q5: what is the total weight of the boat when fully rigged and in the water.
Answer: estimate 1200lbs. (800# hull + 253#eng + batt. 65# + fuel (12@6.5#) 78#.
Plus passengers

Q6,7 no clue. I don't expect prop diameter availability to vary more than 1/2", perhaps 1/4" smaller...usually for high load (ie. work boat)? I haven't investigated what is available through Yamaha or aftermarket.

[Phil T]

Dave -

Dozens and dozens of 16' model (Nauset, Sakkonet, Eastport et.al) owners have selected the Yamaha F70 as their repower choice.

After many rounds of testing, these are the recommended props. Any one of these is a good selection.

13 x 17" Yamaha Painted SS
13-1/4" x 14" Yamaha Performance Series
13-1/4” x 17” Turbo 1
13-1/4" x 14" PowerTech SCP

Notes:
- Size only applies to the make and model listed and is not transferable.
- Engine mounting: Engine is mounted 2 holes up with the top engine bolt in the 3rd hole, counting down from the top.

[Dave_a]

Perfect Phil, your info is in the hands of the tech rigging the engine. The good news is he has a 70 on his boat.

JimH. I gave you all your requested info. Did it result in some prediction of propeller pitch? What are the results?

[jimh]

I didn't know there was going to be a quiz...

In order to calculate a propeller pitch I need all the information I asked you for. I don't rely on archives of anecdotal reports. I figure out the propeller pitch using algorithms that have a history of good outcomes. I don't want to waste time guessing at all the data, so that's why I asked you to tell me the data for your application.

I gave you all your requested info. Did it result in some prediction of propeller pitch? What are the results?

Yes. Here is the predicted propeller pitch based on your input.

For 70-HP and only 1,200-lbs, a 16-foot hull should reach 46-MPH, as predicted by the Crouch Method, and the rounded bottom hull of your older 16-footer will have a hull factor of 190 when using my Crouch's Calculator to do the math.

At 6,300-RPM via a 2.33:1 reduction, with SLIP = 10 to reach 46-MPH will require a propeller pitch of 20-inches, using my Propeller Calculator.

I think the total weight value you provided is too low. It was not clear if you included weight for the crew. I will recalculate with 1,600-lbs, which could be appropriate for two adults and some of their gear, water, and ice.

Top speed will be 39-MPH. Propeller pitch will be 17-inches.

The suggestions that the propeller pitch could be as low as 14-inches seem rather odd. With a 14-pitch propeller, the expected boat speed would be about 31-MPH. That seems very slow for a 16-foot hull with 70-HP.

[Phil T]

Not all 17" pitch propellors are the same. While there are a few select models from different makes that perform in a similar way in the same pitch, they are the exception to the rule.

Many Boston Whaler owners have found that you need to investigate the make and model of prop, not just pitch. It is for this reason I share all the prop identification information so there is confusion.

[jimh]

It is for this reason I share all the prop identification information so there is confusion.

I think you meant to say "less confusion."

Yes, there can be differences in the effective pitch of a propeller compared to its marked pitch, and this certainly varies among the several popular brands.

The only really good way to find out what propeller is going to work is to get one and test it on the boat-engine-load configuration it will be expected to be used.

Another factor affecting engine speed and propellers is engine mounting height. Some propellers have their pitch marked for being mounted very high, and if the propeller is buried in the water it may act like a much higher pitch.

[Dave_a]

A 13.5D x 17P prop is on the boat for test #1. I'll report back results. Engine mounted # 2 hole, cavitation plate is 1/4" above the boat bottom.
It will be 2 weeks or so for results.

[Phil T]

Dave -

You have missed a very important rigging requirement:
From my prior post:

- Engine mounting: Engine is mounted 2 holes up with the top engine bolt in the 3rd hole, counting down from the top.

P.S. Always include the model name and size when referring to a prop.

[Dave_a]

No, I didn't miss it Phil. The tech who rigged the engine to the boat determined that [described mounting height in ambiguous manner] was too high in his opinion.

If the [anti-ventilation] plate was 0.25-inch above the hull bottom in [some other position] then in [a higher position] the cavitation plate would then be 1.25-inch above the bottom of the boat--wouldn't it?

The mechanics's opinion [was for the anti-ventilation plate to be 1.25-inches above the hull bottom] was too high.

Should [the anti-ventilation plate] be 1.25-inches above the hull bottom for best performance?

[Moderator's note: do not describe engine mounting height in terms of some vague number associated with a mounting bracket hole. Engine mounting height is ALWASY to be described in the relation to the lowest possible position for mounting on the transom and using units of "holes" which are spaced at intervals of 0.75-inches. The common engine mounting height is to raise the engine one-hole-up from lowest position or in some cases to raise the engine by two-holes-up from lowest position.]

[jimh]

If the [anti-ventilation] plate was 0.25-inch above the hull bottom in [at some mounting height] then in [a higher mounting height] the cavitation plate would then be 1.25-inch above the bottom of the boat--wouldn't it?

The vertical spacing interval in the pre-drilled engine mounting bracket is only 0.75-inch. Moving an engine "up one hole" only raises the engine by 0.75-inches.

Whether or not an engine works best at a particular engine mounting height is determined by on-water testing. Whether a boat and engine will work best at a higher engine mounting height often depends greatly on the type of propeller to be used and the type of boating to be done.

Another variable in the engine mounting height is the location of the existing holes in the boat transom. In the particular case of older Boston Whaler boats with shallow engine splash wells, the original engine mounting holes in the transom may no longer be useful, depending, of course, on where they were drilled.

Note that in my early reply in this discussion I asked for information on the location of the engine mounting holes as existing currently in the transom, but did not get a reply with that information.

For advice on engine mounting and location of engine mounting holes, see these articles:

Engine Mounting Height Nomenclature
https://continuouswave.com/forum/viewto ... ?f=7&t=739

Considerations in Engine Mounting Height
https://continuouswave.com/forum/viewto ... f=7&t=6618

Transom Mounting Hole Layout for Shallow Splash Well
https://continuouswave.com/forum/viewto ... f=7&t=5808

[Phil T]

Many older generation outboard engine installers and those not well versed with Boston Whalers install the engines too low due to lack of knowledge or sheer laziness. "I do this all day sonny I know what I am doing," is frequently reported.

Dozens and dozens of owners have tested new engines and props on their Boston Whaler boats over the 20 years I have been on this forum and their results are in the archives.

The consensus of opinion and technical testing results recommends a minimum engine height of 1-1/2-inches above the keel of a 16/17-foot Boston Whaler boat. If you ask jimh (owner and creator of this site) if this statement is true, I am sure he would agree.

With a light load and just you, the F70 engine with one of the recommended props above should reach 6100 to 6300 RPM and boat speed will be 37 to 39-MPH in good conditions.

I would suggest you test the engine, finish break-in, and then have that same tech raise the engine on his dime.
You will gain at least 200-RPM engine engine speed and 1 to 3-MPH boat speed as a result of the reduced drag from less of the engine being in the water.

It is your boat, your $10,000 installation. Do what you think best.

[jimh]

The consensus of opinion and technical testing results recommends a minimum engine height of 1-1/2-inches above the keel of a 16/17-foot Boston Whaler boat. If you ask jimh (owner and creator of this site) if this statement is true, I am sure he would agree.

I can't answer with a simple agree/disagree option as suggested. My answer is going to longer and more involved.

The principal reason I can't agree or disagree with Phil's remarks is because I have not personally researched the data available, nor have I had the pleasure of owning a 16/17-foot hull, and I typically would not use the distance between the bottom of the hull at centerline and the engine's anti-ventilation plate as the benchmark.

In my thinking, the goal of the engine mounting height is to allow for the anti-ventilation plate to be riding just at or just above the surface of the water that is flowing over the gear case when the boat is on plane in the upper speed range of it planing speeds and with the engine trim set so the anti-ventilation plate is parallel to the water flow. When the boat is sitting on a trailer and the engine is trimmed to some position which may or may not be where it will be when on plane and trimmed as described above, exactly what the distance will be on a particular boat with a particular engine between the bottom of the hull at centerline and the anti-ventilation will be is really hard to know.

Also, there are more variations in what sets the engine's height, such as where the transom mounting holes are drilled, exactly how long the engine lower unit might be to meet the recommended "shaft height" for the particular engine model, and how far set back from the transom the engine mounting puts the gear case. All of these are variables.

A final variable is the type of use intended for the boat and what is desired. If the boat is going to be run on a lake in mostly calm water and the goal is the highest possible straight line speed, the choice of propeller will be different than if the boat is going to be run on big water in big waves with a heavy load, and the goal is to avoid propeller ventilation as much as possible.

Regarding measuring the distance the A-V plate is above the hull bottom at centerline

If there is some prior discussion in which an installation of a Yamaha F70 is described in detail and includes dimensions for all the variables, and also happens to include a measurement of the distance from the anti-ventilation plate to the hull bottom (when the engine trim is set the same as would be used when on plane), then that dimension might be useful to know. But the problem with even knowing that dimension is you first would have to install the engine at a particular mounting height, then set the trim to be correct (that is, to be the same trim that will be used when on plane), and then measure the distance from A-V plate to hull bottom at centerline. What would happen if your measurement was 3/8-inch off from 1-1/2-inches. The next engine mounting height change would be 3/4-inch, you'd then still be 3/8-inch off from the 1-1/2-inch benchmark.

Right now it seems like the dispute in the particular case of this engine installation is over a change of 3/4-inch in selecting which set of pre-drilled engine bracket holes should be used to location the top bolts. (The bottom bolts will be located by where the bottom holes in the transom happen to be drilled, unless the BIA recommended positions were strictly applied.)

By the way, a "standard" propeller for a 70-HP engine will be a 17-pitch. This is because the typical application for a 70-HP engine is on a boat whose size and weight will permit a boat speed of about 37- to 39-MPH. Assuming the engine speed range permits 6,000-RPM and the gear ratio will be 2.33:1, working out the pitch for 38-MPH gives 17.3-inches.

[Phil T]

Jim -

The phrase "standard propeller" is outdated, inaccurate and not always helpful. If anything, it fosters the continuation of the past sterotyped information.

Propellers are not included by the manufacturer and are sold separately from outboards larger than 30hp.

The F70 has a WOT limit of 6300 rpm.

Just because you have not tested the boat/engine/propeller combination personally does not meant you should distance yourself from information you have read and answers to your questions on the topic over 20 years.

[jimh]

As I said earlier in my introduction to my reply:

The principal reason I can't agree or disagree with Phil's remarks is because I have not personally researched the data available, nor have I had the pleasure of owning a 16/17-foot hull, and I typically would not use the distance between the bottom of the hull at centerline and the engine's anti-ventilation plate as the benchmark.

Those are the three reasons I cannot avow that the magic number is always 1-1/2-inches above keel bottom for the A-V plate on a 16/17 footer.

Phil apparently has researched this, and when I remark that I cannot join with him and declare it so, I am not suggesting his research is in error. I just don’t have a basis to agree or disagree.

Again, the real test is where the water flow line ends up on the gear case when the boat is on plane. We could work backwards from there to discover the engine mounting height that produced that outcome, then we could measure the vertical offset for A-V plate to hull bottom on centerline.