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Old 08 August 2022, 22:13   #1
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Tohatsu 25 EFI 4-stroke vs Tohatsu 18 2-stroke

Here is a comparison of two very different engines installed on my Zodiac Futura Mk2c (3,7m SIB).

I've had the Tohatsu M18E2 for a while now. It is a classic 18 hp 2-stroke that weighs 43 kg I believe. My teenage son wants more power for the Futura and I want a quieter engine. So a few days ago I bought a very nice Tohatsu MFS25B, which is a modern four stroke fuel injected engine with tiller, electric start and gas assisted tilt. It weighs 83 kg. Unfortunately both outboards are long shaft. The Mk2c takes a short shaft but I have built are transom extender. The Futura is rated for 40 hp and 96 kg on the transom so either engine should be ok.

The 25 EFI is a smooth running engine that starts easily even without the electric start (which I don't use). It runs well at any rpm and is pretty quiet even at full throttle. The noise is much easier to handle than that of the M18 2-stroke. The gas assisted tilt is a heavy feature but also a nice luxury that lets me drop the engine without any effort, run shallow mode at any height and makes it easy to tilt up. It is actually very nice!

The raw performance of the 25 is however a bit disappointing. Holeshot feels no better than the 18 (possibly worse?) and top speed is only 2 knots better even though I had two suitable props to try out. 22 vs 24 knots with a total load of 250 kg. I was hoping for 28 knots or so with the bigger 3-cylider engine, but no such luck.

The real issue with the 25 is that it is a heavy beast. It is no longer portable. I can't carry it around alone. The SIB is really hard to pull out of the water with the larger engine. Before my 14-year old son could just barely do it alone. Now it takes two people. The boat alone weighs 97 kg. The worst thing is that the Futura can't really handle the weight. The whole transom flexes and the boat porpoises constantly when on plane regardless of weight distribution. I'm sure my transom extender doesn't help here but I can see the whole transom flex with the porposing.

All in all, I think the 25 hp Tohatsu EFI is a great outboard! On another boat. It's just too heavy for a SIB. And too bulky to be portable, it takes up half of the trunk in the car with the seats folded. The smaller M18 is still portable and suits the small Futura much better. But it is noisy. Perhaps an old 30 hp 2-stroke is the best outboard for this boat?
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Old 09 August 2022, 10:11   #2
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That's a very interesting and telling review. It supports my personal theory that the comment "buy the largest/most powerful outboard the SIB is rated for" is not always correct. As long as you aren't running a heavy load I find the sweet spot for performance and a SIBs feel is often an outboard size down from what many would think.
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Old 09 August 2022, 12:17   #3
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Tohatsu 25 EFI 4-stroke vs Tohatsu 18 2-stroke

Get the correct shaft length and then see
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Old 09 August 2022, 13:01   #4
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Yes, a short shaft would help a bit with the flex I'm sure. But the weight will be nearly the same. I really want to find a good short shaft motor, but that is not an easy task.
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Old 20 August 2022, 19:33   #5
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I have done the same with Yamaha 15hp in 4 stroke (51kg) and 2 stroke (36kg) guise.

I had exactly the same issues. The 4 stroke was more difficult to get on the plane and in any kind of chop it made the boat flex and it feel like the transom was going to be ripped off. The boat was correctly inflated to the correct pressure and checked with an independent gauge.

Weight was a pain in the arse on the 4 stroke, and you can't stow it as easily.

I don't think I would ever buy a 4 stroke engine again unless I had a large trailered boat.
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Old 21 August 2022, 14:06   #6
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The 4 stroke was more difficult to get on the plane and in any kind of chop it made the boat flex and it feel like the transom was going to be ripped off.
A 15hp efi Mercury on a 3.6 sib, and I can honestly say I've never suffered any of those issues.
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Old 03 October 2022, 20:34   #7
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Just to follow up here. The transom developed a (fatal) crack. Probably from using the transom extension and a heavy engine. More can be read in another thread. The purposing actually got sorted pretty well by running the Tohatsu 25 at the lowest trim setting, sacrificing a little speed. Loved that smooth engine but sold it a couple of weeks ago.

Today I picked up a 2005 Johnson 30 hp 2-stroke. It is about as light as it gets for a 30 hp engine (52 kg) and is still reasonably portable. It was a little pricey at 1500 EUR but it is in good shape and compression is good (135/138 psi). Johnson is not my preferred brand but at least it is short-shaft tiller 30 hp and only a hour away. And it came with a small RIB and a basic hand trolley. The RIB is an Arimar Top-line 320 in working condition. The seat cushion is ripped but apart from that it just needs cleaning.

But fall is here and we'll see when I get a chance to try the new motor.

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Old 03 October 2022, 21:34   #8
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So is the plan to fit a new transom to your Zodiac and use it with your new to you 30hp Johnson?
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Old 04 October 2022, 11:56   #9
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dhaglund's transom thread:
http://www.rib.net/forum/f50/zodiac-...for-88873.html
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Old 04 October 2022, 12:14   #10
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I've read and commented on that, but it doesn't answer the question, which I asked for specific reasons.

Going from 18, 25 to 30 on a knackered, albeit repaired transom, may cause more damage.

If he is going to use it on his newly acquired Rib, according to the specs online that RIB is only rated at 15hp.
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Old 05 October 2022, 07:51   #11
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So is the plan to fit a new transom to your Zodiac and use it with your new to you 30hp Johnson?
For now, the plan is to reinforce the current transom and use the 30 hp Johnson. An older, light 25/30 2-stroke seems to be the optimal engine for a Zodiac Futura IIc if you want as much power as possible while keeping the weight down. Sooner or later that boat will need a new transom though.

The Arimar Top-Line 320 has a max rating of 15 hp (as you already stated) so I only bought this package for the engine. But the previous owner has successfully used that motor and boat together for years. It did require hydrofoils however. Topping out at 30 knots and able to plane with 4 adults

I'm not too worried about more transom damage or even glue failure on the Zodiac. I got that boat for next to nothing. And I really like it, but if I have to replace the transom eventually, do more gluing or even scrap the boat that is ok.
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Old 05 October 2022, 08:03   #12
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Hope it all works out for you
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Old 05 October 2022, 09:52   #13
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Hope it all works out for you
Thank you. We'll see. My son wants as much power as possible to be able to tow a tube and such with his friends. As for myself, I think it would be interesting to see how the Futura would perform with an engine in the same class as yours. Perhaps a Suzuki/Tohatsu/Mercury 20 hp EFI would work great for the most part? It would certainly be quieter and use less fuel.
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