Engine Brand Choices: Effect of Reduced Power on Performance

[jjacobs]

My 1973 13-footer needs a new outboard engine. The c.1975 Evinrude 40-HP outboard engine that powered it has stopped working.

In my opinion all the current 40-HP engines are too heavy, so I'm thinking of re-powering with a 30-HP or even a 25-HP engine.

Q1: having been used to a 40-HP engine on my 1973 13-footer, will re-powering with a 25-HP or a 30-HP engine make me happy?

Recommend an engine to re-power a 1973 13-footer that was powered by a 40-HP two-stroke-power-cycle 40-HP engine.

Q2: among the following brands listed below which are recommended?

• Mercury
• Tohatsu
• Suzuki
• Honda
• Yamaha

Q3: can you recommend some other engine brand, other than Mercury, Tohatsu, Suzuki, Honda, or Yamaha?

[jimh]

Q1: having been used to a 40-HP engine on my 1973 13-footer, will re-powering with a 25-HP or a 30-HP engine make me happy?

Assuming that the new 30-HP engine weighs as much as the original 40-HP engine, the effect will be reduced boat speed by a factor of about 0.86. This is based on the general theory of naval architect George Crouch, who showed that the speed of a moderate-speed planing hull boat was related to the power-to-weight ratio to the 0.5-exponent. See my calculator that implements this theory at

Crouch's Calculator
https://continuouswave.com/calculators/crouchCalc.php

If the maximum boat speed with 40-HP was about 40-MPH, then the new maximum boat speed with 30-HP would be about 36-MPH.

Assuming the new engine is a modern outboard engine, probably a four-stroke-power-cycle engine, and the old engine was (most likely) a two-stroke-power-cycle engine with simple carburetor fuel induction, the rate of fuel consumption with the new engine will be noticeably lower, and particularly much better at lower throttle settings, so the over-all fuel usage will reduce by almost half.

If the reduced top speed reduces the degree of happiness but the reduced fuel consumption increases the degree of happiness, the outcome may be a net zero change in happiness, other than the decrease in happiness from the expense of the new engine.

The other benefits of the new engine will be faster starting, less exhaust smoke, and often a perception of lower noise.

The other detriments of the new engine may be a smaller power band at which the rated power is produced, leading to slower acceleration and more reduction in performance as total boat weight increases, as when additional people are aboard.

[jimh]

Q2: among the following brands listed below which are recommended?

• Mercury
• Tohatsu
• Suzuki
• Honda
• Yamaha

Q3: can you recommend some other engine brand, other than Mercury, Tohatsu, Suzuki, Honda, or Yamaha?

Your list of engine brands encompasses all the major brands, so I do have any other brands to suggest. All those outboard engine manufacturers make good engines.

My general advice about which engine brand to buy is not particularly determined by the brand itself, but rather more influenced by the dealer that is selling and supporting the brand. All engines will need some after-sale support, routine service, possible repair under warranty or out of warranty, and typically most of this support will be provided by the dealer that sold the engine. On that basis, I would make the dealer, his reputation, his history in the outboard engine business, his repair shop, the technicians he employs, the relative location from where the boat will be used or kept, and your own impression of the dealer to become important considerations in which brand to buy.

And, of course, cost and availability are always a factor.

After you see the cost of a new 30-HP outboard engine, you may take a greater interest in trying to repair the original 40-HP engine. Unless the reason the engine no longer runs is due to a catastrophic failure of a major component, throwing $1,000 into repair of the old engine might sound better than spending $4,000 on a new engine.

[Jefecinco]

Our 1981 Sport 13 is powered by a 30 HP EFI Mercury with Tilt & Trim. At WOT, correctly trimmed for maximum speed it is a little scary. We are a boating couple so with three or more aboard it may be slightly slower.

[jimh]

As I believe you are already aware, the modern, large Tohatsu engine manufacturing plant in Komagane, Nagano Prefecture, in Japan makes the engine Mercury sells.

My recollection of the smaller four-stroke-power-cycle Tohatsu engines is that they have some sort of throttle body fuel injection and they can be started and run without a battery. Those are both very nice qualities. I think Tohatsu concentrates on these smaller horsepower engines. There are interesting discussions in the archives at

https://continuouswave.com/forum/viewtopic.php?t=6290

and at

https://continuouswave.com/ubb/Forum4/HTML/003389.html

that talk about Tohatsu engine, and their high reliability. You really need to read all of that second link's long thread.

[jjacobs]

Thank you all for your advice. I've chosen to buy a Suzuki 25-HP for a good dealer, available in short shaft, and lighter than my current 1983 Evinrude 40-HP engine.

Another question:

Q4: would a 1983 Evinrude 40-HP short-shaft outboard engine that appears to be seized have any value for someone needing a short-shaft 40-HP and willing to rebuild it?

The dealer selling me the new engine plans to junk the old Evinrude. I hate to junk this old outboard engine if it can be useful to someone.

[jimh]

Q4: would a 1983 Evinrude 40-HP short-shaft outboard engine that appears to be seized have any value for someone needing a short-shaft 40-HP and willing to rebuild it?

My local Evinrude dealer--selling Evinrude engines for over 100-years--has a back room full of old engines that have some critical flaw, but they all have plenty of good parts. If you can find a similar long-time Evinrude dealer around your area, perhaps he might have some interest in your 1983 40-HP engine, maybe not for a cash sale, but at least to keep it out of a dumpster. There is also an antique outboard engine website, and you could advertise you have this 41-year-old 1983 outboard engine available. There might be someone interested in it as a project or as a source of parts. See:

Antique Outboard Motor Club Classified Ads
https://www.aomci.org/classifieds/

[jimh]

I've chosen to buy a Suzuki 25-HP...

After you get the new engine installed and dialed-in with the proper propeller, let readers know the new top boat speed with the 25-HP.

According to the Crouch Speed prediction formula, a decrease in power to 25-HP from 40-HP should cause the top boat speed to decrease by a factor of (25/40)^0.5 = 0.79. Not knowing exactly what your top boat speed was, let's say it was 40-MPH; with the reduced power the top boat speed would be down to 31.6 -MPH, as long as the total boat weight remains the same. If the total boat weight is less, then the speed with 25-HP would increase a slight amount.

[jjacobs]

Thank you all for your advice during my decision process.

I have now had the SUZUKI 25-HP engine on my 1973 13-footer for about 10 hours of run time.

Here are my observations:

• The SUZUKI 25-HP four-stroke-power-cycle engine has as much get up and go as the old Evinrude 40-HP two-stroke-power-cycle engine. This was a surprise. With only the operator in the boat, top speed is scary and the boat tends to skip across the water, losing some of its side to side grip.
• Top boat speed is about 25 [assumed that the units are statute miles per hour or MPH].
• The SUZUKI 25-HP four-stroke-power-cycle engine is a little lighter than old Evinrude 40-HP two-stroke-power-cycle engine, but the power-tilt mechanism takes up some space between the transom and the power head, thereby shifting the weight farther back. As a result, the boat has a high-nose attitude as it accelerates to planing. I haven't found it to be a terribly big deal, and shifting my own weight forward as well as moving the gas tank forward corrects the problem.

[jimh]

There are some incongruities in your observations.

First, if a 25-HP four-stroke-power-cycle appears to have the same power as a 40-HP two-stroke-power-cycle engine, the only reasonable explanation was the 40-HP was not running properly, may have been running on with one dead cylinder, or in some way was not producing anywhere near its rated horsepower. A classic 13-footer with a 40-HP would have been much faster at full throttle than the 25-MPH you are reporting for the 25-HP engine.

Second, if the actual weight of the 25-HP four-stroke-power-cycle is less than the weight of the older 40-HP two-stroke-power-cycle engine, there should not be much effect on boat trim, and certainly not a tendency for the trim to become bow high.

A more reasonable explanation for the appearance of a new behavior (greater bow rise in transition to on-plane) is that the 25-HP four-stroke-power-cycle engine cannot produce much power at lower engine speeds, and it takes a longer time to accelerate onto plane, causing the bow rise to be more noticeable. This sort of outcome is what everyone experiences when they switch from a two-stroke-power-cycle engine (in proper tune) to a [naturally aspirated non-supercharged) four-stroke-power-cycle of the same rated horsepower, and that you are switching to a 25-HP from a 40-HP, which is a decrease in power by a factor of 0.625 or a 37.5-percent decrease in power, it would be just about inevitable that the time to plane would be slower and more labored.