1963 13-footer

Optimizing the performance of Boston Whaler boats
poodleboater
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Joined: Sun Feb 14, 2021 9:57 pm

1963 13-footer

Postby poodleboater » Sun Feb 14, 2021 10:14 pm

For a 1963 13-footer I will buy a new engine; I plan to decide among Yamaha 25-HP, Mercury 30-HP, and Yamaha or Mercury 40-HP engines.

I have seen many boats [presumably 13-footers] with 40-HP engines, but the weight [of a 40-HP engine] seems to cause the boat's static trim to settle a bit to the stern. I've only seen this in pictures. I know that installing the woodworking farther forward and moving the battery forward can help [reduce the effect of engine weight].

Q1: can excess engine weight still be an concern?

Q2: would a 25 or 30-HP engine have enough power for my demands?

Q3: would the 30-HP engine be of a significant advantage over the 25 hp engine?

Q4: would a 25-HP and 30-HP engine perform very similarly?

I am open to any other suggestions. I am open to hearing any information from anyone willing to share.

I plan to get the engine installed in April or May 2021. I hope to order the new engine ahead of that time.

BACKSTORY: I bought a 13-footer that had been converted to a fishing boat. I'm in the process of restoring it, and am currently waiting on my new mahogany to arrive and some less frigid temperatures. I intend to install Nauset Marine's transom riser and wedge. Although I don't personally fish, I'm interested in a boat that can carry between one and four people, and can tow tubers or skiers. I understand there isn't a perfect solution for all these uses. I'm looking for the best compromise.

jimh
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Re: 1963 13-footer

Postby jimh » Mon Feb 15, 2021 11:08 am

poodleboater wrote:Q1: can...engine weight still be an concern?

Yes, engine weight is always a concern, and perhaps more than ever on a small boat like a 13-footer. The smaller the boat and the lighter the hull construction, the more added weight like an engine will affect the trim on the boat.

The notched transom of the older 13-footer reduces the freeboard at the stern. I don't know if the re-fit with the Nauset Marine's transom riser and wedge products will completely seal out ingress of water over the original transom. However, the original 13-footers had a deep engine splash well, and that splash well should keep water coming over the transom confined to the splash well. On the other hand the deep splash well full of water adds even more weight at the stern. As does your transom riser and wedge products.

Image
Fig. 1. An older 13-footer with the notched transom and deep splash well.

jimh
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Re: 1963 13-footer

Postby jimh » Mon Feb 15, 2021 11:14 am

poodleboater wrote:Q4: would a 25-HP and 30-HP engine perform very similarly?

Presuming that at the propeller shaft there is a difference in power of 5-HP and no change in weight, then the 30-HP engine will push the boat faster at full-throttle than the 25-HP engine.

Boat speed is related to the ratio of power to weight to the square-root of that ratio. Assuming no weight change, then a ratio of (30/25)^0.5 is 1.09, which means the boat speed would be 9-percent higher with the higher power. For example, if the boat could accelerate to 25-MPH with the 25-HP engine, then with the 30-HP of the same weight the boat could accelerate to 27-MPH.

Whether or not a variation of 2-MPH at full throttle qualifies as "perform very similarly" is subjective to the boat operator. In the case of my own boat, the top speed of the boat often varies by 2-MPH due to the influence of the weather, the amount of weight being carried, the state of the sea, and other factors. Thus, for me, for my boat's top speed to vary by 2-MPH is not what I consider dissimilar performance or indicative of a need to increase the engine to greater power.

Any increase in weight will reduce the amount of increased speed. If the added weight of 30-HP keeps the power-to-weight ratio the same as with 25-HP, there will be no change in top speed.

jimh
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Re: 1963 13-footer

Postby jimh » Mon Feb 15, 2021 11:27 am

poodleboater wrote:Q2: would a 25 or 30-HP engine have enough power for my demands?


To answer this, your "demands" must be established. I believe your "demands" include towing water skiers.

WATER SKIING

I doubt you can tow more than one water skier at a time with a 13-footer and a 25, 30, or 40-HP engine.

I doubt you can tow a large adult water skier who wants to get up on one ski. If you are thinking about pulling up a 250-lbs guy, you don't have the right boat.

You can certainly tow a smaller adult water skier or young person water skier. I have seen a 13-footer with a 25-HP engine get a 25-year-old woman up on two skies, but that was a rather thin and athletic person that maybe weighed 115-lbs.

CARRYING FOUR ABOARD
I believe your "demands" include being able to carry four people aboard. A 13-footer can handle four people, based on the capacity sticker rating that allows six people. See

http://continuouswave.com/whaler/reference/13/

As I mentioned in my earlier comment about horsepower and weight, adding weight reduces speed. In the case of small and light boats like a 13-footer, the difference in performance with one person aboard compared to with four persons aboard will likely be very significant. Three adults may add 600-lbs to the total boat weight--more than double the hull weight--and it is certainly going to be a significant increase in the total weight.

When there are four aboard a 13-footer, the performance will decrease. The top speed will decrease. Because the boat is a planing hull, the ability to get on plane will be reduced. Whether or not the boat can still accelerate and transition onto plane with a particular horsepower is hard to predict. It will depend on the power-curve of the engine and the actual weight of the people. The transition onto plane often requires more power than to remain on plane. Whether a particular engine can develop enough horsepower while limited in engine speed to accelerate by the load is difficult to predict. In general, older two-stroke-power-cycle engines were able to produce more power at lower engine speeds that the typical modern four-stroke-power-cycle engine.

NOT INTERESTED IN FISHING
I believe another of your "demands" is the boat will not be using for recreational angling. I think a 13-footer is a very useful boat for purposes other than for recreational angling.