Menemsha 16 Power to Reach Plane

Optimizing the performance of Boston Whaler boats
menemshaman
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Menemsha 16 Power to Reach Plane

Postby menemshaman » Tue Oct 19, 2021 12:37 pm

I plan to use a MENEMSHA 16 for fishing and pleasure cruising. I want the boat to be able to get to planing speed with two adults, two children, and a dog aboard. Once the boatspeed reaches hydroplane I don't want to go any faster than 30-MPH. I intend to weigh the hull. The Menemsha has more weight forward with the cuddy [than other 16-foot models have].

Q1: does the forward cuddy's weight change the boat's center of gravity?

Q2: does the forward cuddy's weight help or hinder the hull reaching hydroplane speed?

Q3: what is the minimum engine horsepower that can propel the hull to plane with a added load of 1,200-lbs greater than the bare boat weight? I believe the bare boat weight will be 1,200-lbs.

Q4: at what boat speed does a 16 MENEMSHA reach plane with proper weight distribution?

Many thanks for any and all input.

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Phil T
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Re: Menemsha 16 Power to Reach Plane

Postby Phil T » Tue Oct 19, 2021 5:31 pm

The cabin [weight] does not make much difference.

Presuming the proper engine height and a stainless steel propeller I would expect the hull to plane at 12 to 14-MPH.

In your engine calculations, keep in mind it is not just about top speed. You also need to factor in engine torque.

From time to time I ran with three to four people aboard with gear, luggage, freight. Maintaining a moderate to low cruising speed in ocean waves, swells, current and a bit of wind took every bit of the maximum horsepower for that hull.

An 90-HP would be a practical choice given a hull weight of 1,000-lbs, engine weight of 450-lbs, 50-lbs of battery and anchor, and 1,000 lbs of persons, fuel and gear..

[If your rig with a 90-HP and] if you decide at a later date to sell the boat, you will not be penalized for a low horsepower boat.
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menemshaman
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Re: Menemsha Minimum HP For MY Use

Postby menemshaman » Tue Oct 19, 2021 9:41 pm

Thank you Phil T for your detailed response.

Regards,

Kent

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Re: Menemsha 16 Power to Reach Plane

Postby jimh » Wed Oct 20, 2021 2:14 pm

menemshaman wrote:Q1: does the forward cuddy's weight change the boat's center of gravity?
Yes, but so does the engine, the fuel, the battery, and the people aboard

menemshaman wrote:Q2: does the forward cuddy's weight help or hinder the hull reaching hydroplane speed?
All added weight reduces performance, so the weight of the cuddy reduces performance compared to an identical boat without the cuddy.

menemshaman wrote:Q3: what is the minimum engine horsepower that can propel the hull to plane with a added load of 1,200-lbs greater than the bare boat weight? I believe the bare boat weight will be 1,200-lbs
You can use the Crouch method to predict boat speed based on horsepower and weight. I have create a calculator to assist you. You can assume the hull constant for a classic non-smirk 16-footer is 180. Visit

https://continuouswave.com/calculators/crouchCalc.php

Enter the weight, 2,400, and the speed at which you think the hull will reach plane. I would guess that 25-MPH is probably a true planing speed. The calculator will predict the power needed to be 46-HP. However, this can be misleading because to reach plane there must be more power than that needed to push the boat up the bow wave.

The original specification for the 16-foot hull (not a Menemsha) to reach plane with a very light load was 35-HP. The added weight of the cuddy that will require more horsepower. The added 1,000-lbs (over the suggested light weight) will required more horsepower.

menemshaman wrote:Q4: what boat speed does a 16 MENEMSHA reach plane with proper weight distribution?

I don't understand the metric "proper weight distribution." Weight distribution affects the hull trim. A boat in “proper trim” will have a neutral fore-aft static trim to a slightly bow-high static trim.

menemshaman wrote:Many thanks for any and all input.

You are welcome.

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Re: Menemsha 16 Power to Reach Plane

Postby jimh » Wed Oct 20, 2021 2:19 pm

Phil T wrote:An 90-HP would be a practical choice...

There is no doubt that a 90-HP will be a very practical engine for a Menemsha with four aboard and a dog.

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Re: Menemsha 16 Power to Reach Plane

Postby menemshaman » Wed Oct 20, 2021 7:02 pm

Wow now that is an answer with some meat to it! Thank you so much Jim.

Regards,

Kent

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Re: Menemsha 16 Power to Reach Plane

Postby jimh » Thu Oct 21, 2021 8:25 am

The Crouch method can also be used to deduce the boat speed that would occur with the suggested minimum power of 35-HP specified by Boston Whaler for this hull and a "light" load. Here assuming the empty 16-foot hull weight would have been 550-lbs, the engine would have been 300-lbs, and a crew weight would have been 180-lbs, we have already 1.030-lbs. Let's add some more weight for a battery (50-lbs), fuel (50-lbs), some seating (50-lbs), and some miscellaneous gear (20-lbs) to get a total "light" weight of 1,200-lbs. We use that weight as the input to the calculator. WIth a hull factor of 180, with 35-HP the boat speed would then be 30.7-MPH.

As I mentioned earlier, to get onto plane there must be enough power to push the boat uphill and onto the bow wave the hull creates. The bow rise during transition to plane is created by climbing up this wave. If the boat does not have enough power to climb up and over its own bow wave, then it will not be able to get onto plane. Once the boat is on plane, it can usually maintain plane with less power than was needed to get onto plane.

If a particular boat and engine combination is always going to be operated with a heavy load and with marginal engine power, the propeller pitch can be lowered. With less pitch, the propeller will be easier to turn, and with marginal engine power this may allow for the boat to reach plane, at the sacrifice of eventual top speed.

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Re: Menemsha 16 Power to Reach Plane

Postby goldstem » Thu Oct 21, 2021 11:29 am

Minor points:
—a 35 HP outboard does not weigh 300 pounds. A 35-HP engine available in the era when the 16's were made weighed about 140 pounds ;

—our 16-footer with just a light aftermarket console and no seats tops out at around 31-MPH with a new 50-HP engine.

I have a hard time believing a powerhead-rated 35 with the added [weight] of the cabin could reach 35. Boston Whaler estimated a Nauset with 40 hp would top out at 28 in 1970.

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Re: Menemsha 16 Power to Reach Plane

Postby menemshaman » Thu Oct 21, 2021 5:19 pm

Thank you again Jim and thank you Goldstem. I have come to my Menemsha from sailing so I am a newbie in the truest sense. I have had two power boats, an 11 foot tin Jon boat with a 3 horsepower Johnson 2 stroke and I currently have a 15 foot dory with a new 20 HP Tohatsu. I Loved the Jon boat and I Love my dory. Many of my friends have larger power boats and enjoyed water skiing and tubing. None of that interests me. The Menemsha now has a fine running 4 stroke 90 HP 2002 Mercury and I am finding it to be more motor than I need and want. I am considering a new 60 HP Tohatsu. I am considering this motor for the following reasons;

1) I own 2 new Tohatsu motors (sailboat kicker and dory) and have had great luck with them. I want a new motor for the Menemsha. I like the 5 year warranty for peace of mind.
2) I want a smaller, less powerful motor that I feel will better suit my use and boating style. I also save about 100 pounds also going to a smaller, lighter motor.
3) My research and the valued advice I have been given from generous forum members tells me that this size motor will be more than adequate to get the Menemsha on plane and run well at a moderate speed. My dory goes about 25 MPH (My guess at this point as I haven't checked it) and that is more than fast enough for my purposes and pleasure.

So will the 60 HP Tohatsu fill the bill?

Thank you all for any more sage advice.

Regards,

Kent

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Re: Menemsha 16 Power to Reach Plane

Postby jimh » Fri Oct 22, 2021 10:48 am

goldstem wrote:I have a hard time believing a powerhead-rated 35 with the added [weight] of the cabin could reach 35 [MPH].

More minor points:
  • in the example above which predicts a speed of 30.7-MPH (not 35-MPH) the power used is propeller shaft power of 35-HP
  • the boat used is the lightest possible 16-foot hull, not a Menemsha
  • if a lighter engine weigh were used, the predicted speed would increase.

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Re: Menemsha 16 Power to Reach Plane

Postby jimh » Fri Oct 22, 2021 10:58 am

goldstem wrote:...our 16-footer with just a light aftermarket console and no seats tops out at around 31-MPH with a new 50-HP engine.


Assuming the boat weigh of your 16-foot bare hull with "light" console is about 1,200-lbs, if a 50-HP engine can only propel the boat to 31-MPH then the hull factor calculates to just 152. That figure is very far out of the normal range for a classic Boston Whaler hull. On that basis, there must be some error in the data.

Using the hull factor of 180 produces a calculated top speed of 36.7-MPH for 50-HP and 1,200-lbs. You're missing 5-MPH. Or, the boat weighs much more than 1,200-lbs. Or you are using nautical-miles-per-hour, thus 31-nautical-miles-per-hour is actually 35.5-MPH. If the unit of speed is different than MPH, please be clear that you are using another scale for boat speed.

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Re: Menemsha 16 Power to Reach Plane

Postby goldstem » Mon Oct 25, 2021 10:26 am

The major negative factor in our maximum boat speed is that we currently run the large, 'high - thrust' gear case. In our usage this was a tradeoff worth making. A prior engine with the small gearcase reached about 34mph max speed.

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Phil T
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Re: Menemsha 16 Power to Reach Plane

Postby Phil T » Mon Oct 25, 2021 12:18 pm

Kent -

The guys are getting distracted and not looking at the original question.

A flat bottomed Jon boat and non-displacement Dory are not comparable a 16'7 cathedral hull weighing ~700-800lbs with a 100lb fiberglass cap on top.

You reference an additional 1200 lbs of weight in the boat. [See Q3]

1200 lbs is ~6-7 people. If you put that many people in a Nauset or other 16'7, the boat will not get out of displacement mode and be limited to ~12-14 mph with anything less than a 90hp.

If you are in the ocean or any body of water with current or waves, you may not even plane off even with the 90hp.
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menemshaman
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Re: Menemsha 16 Power to Reach Plane

Postby menemshaman » Mon Oct 25, 2021 8:05 pm

Hi Phil:

Thanks for your input. I have read that the bare hull should weigh about 1,300 lbs. Add 220 lbs for a 60 HP Tohatsu, 75 lbs for 12 gallons of fuel, 500 lbs for people and dog, plus 300 lbs of water in the hull (Guessing at this point, fingers crossed) equals 2,395 lbs. Add 105 lbs of miscellaneous gear and you come up with 2,500 lbs total. Using the calculator that should propel the boat at 27.9 MPH. I guess the question is will the boat be able to ride over its bow wave and achieve plane so it can get to this speed.

I used to think that sailing was complicated : )

Regards,

Kent

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Re: Menemsha 16 Power to Reach Plane

Postby jimh » Tue Oct 26, 2021 7:42 am

KENT--if you choose the propeller for the 60-HP engine so that the propeller pitch is appropriate for a top boat speed of 28-MPH, the engine should be able to accelerate and put the boat on plane.

If we knew the gear ratio and the manufacturer's maximum engine speed for the "60-HP" engine, then a propeller pitch could be estimated using a Propeller Calculator.

For example, if the engine top speed is 5800-RPM and the gear ratio is 2:1, then to hit 28-MPH with a SLIP of 10, a propeller pitch of 11 is calculated.

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Re: Menemsha 16 Power to Reach Plane

Postby menemshaman » Tue Oct 26, 2021 7:51 am

Hi Jim:

I had no idea that there would be such choices. I know that people who want to perfectly optimize performance go to great lengths to do so, but I figured it would be more cut and dry to just to buy a stock motor adequate for my needs. I am glad I am getting educated now so I can get the right motor set up properly before I buy and then have to go back and do alterations. Once again thanks for all the great advice!

Regards,

Kent

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Phil T
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Re: Menemsha 16 Power to Reach Plane

Postby Phil T » Tue Oct 26, 2021 3:01 pm

Last comment from me.

If you purchase a 60-HP engine and you can't plane, you will take the proverbial bath (financially) selling it and buying a 90-HP.

If you buy a 90-HP, you will have to focus on something else because all of this is moot. Just because you have a 90-HP does not mean you have to go fast.

I am done.
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Re: Menemsha 16 Power to Reach Plane

Postby jimh » Thu Oct 28, 2021 7:10 am

menemshaman wrote:I had no idea that there would be such choices [in propeller].


KENT--you may find a better explanation of how the load of a propeller affects the ability of an engine to accelerate under that load by reading my article

Propeller Power Curve
https://continuouswave.com/whaler/reference/propellerPowerCurve.html

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Re: Menemsha 16 Power to Reach Plane

Postby menemshaman » Thu Oct 28, 2021 7:46 am

Hi Jim:

Thank you for this article and the work you put into developing it. The outboard shop I anticipate buying my new motor from says he won't receive the shipment from Tohatsu until April or May so I have time to do my due diligence.

My decision to go from displacement sailboats to power boats was to get back to my roots when things were simpler, and I spent all my time on the water instead of all my time working on boats. I bought a 15 foot dory and then the Menemsha.

I was thinking the Menemsha was akin to the dory; a planing hull. I am discovering that is not entirely true and I am fine with that. The Menemsha talked to me and I am committed to making her work for my use.

I am a bit perplexed by the bigger-is-better attitude for engine size. A previous comment advocating a larger engine stated basically that you you can just throttle back and not go as fast: why buy a smaller engine that may serve the intended purpose?

Doesn't weight and cost and fitting the boat with the proper size engine for its intended use matter?

I guess for me, God is in the details.

When I bought my sports car, I rejected many faster cars because they were over-powered and went too fast for what I wanted.

My friend’s Corvette has [an automatic transmission with eight] speeds and will go 65-MPH in first gear, and get there in a split second.

I wanted something tamer, that accelerated briskly, handled really well, and that I could run through a couple of gears before the tickets came. That is the car I bought. After seven years I still Love to drive it.

Perhaps in my analogy I digress.

The same principle applies to my approach to re-powering the Menemsha. It now has a 90-HP engine, and I am not really happy with it. I would be much happier with a smaller engine that will drive the boat to planing speed under most conditions, and wide opeN not go as fast. That is the holy grail I seek.

My preliminary research tells me that A 60-HP engine might be the answer, and give more-than-adequate power to achieve planing speed under normal conditions. An individual, who seems respected (Tom Clark), responded to another post that he knew that a 50-HP engine with a normal load of provisions and people performed very well and easily planed the boat. I added 10-HPfor safety sake.

Maybe I am right and maybe I am wrong.

I thank you and everyone else who has and will help me find the answer.

—Kent

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Re: Menemsha 16 Power to Reach Plane

Postby jimh » Thu Oct 28, 2021 9:32 am

KENT--if you want to use an analogy to a passenger car for a boat trying to accelerate onto plane, the passenger car will have to be operating in a continuous uphill grade, with the initial uphill grade being very steep, then later the uphill grade reduces somewhat, but the car continues forever running uphill.

That is the best analogy for a car to what a boat hull is doing when it is being pushed onto plane. First the boat has to climb over its own bow wave. The height of the bow wave keeps increasing as the boat speed increases, so climbing over it requires much more power than will later be needed to maintain the boat on plane. Once the boat climbs up its bow wave and begins to run on plane, the climb angle reduces, but the boat remains in a position that to compare with a car is still an uphill climb.

The typical car engine at highway speeds will be operating at below 2,000-RPM because of the transmission gearing.

The typical outboard engine on a boat will be operating around 4,000-RPM when the boat is on plane.

If your engine has to run at full-throttle to keep the boat on plane, it will be running at 5,500-RPM or more all the time the boat is on plane--that is a much higher load than is put on a car engine.

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Re: Menemsha 16 Power to Reach Plane

Postby R DAVIS » Thu Oct 28, 2021 11:18 am

jimh wrote:….to get onto plane there must be enough power…
That's a good, clear explanation of [the power needed to push a boat onto plane].

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Re: Menemsha 16 Power to Reach Plane

Postby menemshaman » Thu Oct 28, 2021 5:57 pm

Thanks Jim for an excellent explanation. I appreciate you taking the time and effort.

Regards,

Kent

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Re: Menemsha 16 Power to Reach Plane

Postby fno » Wed Nov 10, 2021 9:56 pm

Except for the small fact that when it is time to sell, for yourself or your heirs they will be offering up an Albatross of a boat. If the boat will remain in the family for generations due to sentimentality, go for it. I for one would never operate any 16' Whaler hull with a 60 HP motor even if it was given to me for free. I would take advantage of the free boat to get at minimum a 75 HP engine or a 90 HP engine. I guess I don't see the advantages of going as slow as possible on plane in a powerboat just for the "fun" of it.