Honwave T38 Transom height and motors

[ChrisParish]

More importantly the AV plate is bigger at the front than the Mariner / Mercury / Tohatsu engines. I really think that is the issue as my Mariner is 17 inches from transom clamp to plate but ventilates like a colander.

@Bigsofty I notice you are trimmed all the way in. Is that your regular running position on your T38?

[Bigsofty]

Hi Chris,
Used the Honda for the first time yesterday (have had a long standing injury after buying it ) and set it trimmed up 2 holes as a starting point. It cavitated really badly at only 1/3rd throttle so trimmed it in all the way which cured it.
Also had a lot of air under the floor but not sure if the trim contrbutes to that issue or not? Time for a thread search

[Bigplumbs]

I am sort of coming to the conclusion you have to be trimmed all the way in. I know on my larger boats with electric trim a very small blip on the switch up will cause a huge cavitation. If you then blip it down you get instant bite

[warder54]

Not on a Honwave, but someone asked for a Tohatsu shaft length earlier. 2018 20 efi measures at 16 3/4" transom to cav plate

[harrison]

Some useful data getting posted on here, cheers.

Whilst not a witch hunt on outboard leg lengths and AV plate profiles, it is interesting to see the variations between makes. So far looks like the new Mariner is the runt of the litter.

Personally not of the opinion every manufacturer except Suzuki has the wrong AV profile, although like the extra width near the leading edge. In reality most outboards work fine on most boats...fact. My understanding of the AV plate is to stop air being sucked down to prop and provide some lift. Assume size kept to a minimum to reduce drag.

It’s not all about the motor though, and think Honwave may shoulder some of the blame here. Yep, the current Mariners are a bit shorter than the norm in the leg, but also the T38 has a deeper than normal transom at a whopping 16.5”. Almost as if they’ve fitted a standard 15” transom and added the floor thickness to it! It also sports generously sized trim tabs, which on such a light boat are largely superfluous to performance, and arguably may even contribute to the problem. Perhaps even elevating the transom further out of the water and resulting with disturbed flow over the prop, particularly if floor pressures drop slightly.

Out of all the sibs we’ve had over the years I can’t recall one where we’ve not had to raise the motor on packers to get optimum performance. Most seemed best with AV plate between 0-20mm below keel. No such luxury on a T38/Mariner 20 combo!

It’s becoming increasingly apparent many T38 users, with various makes of motors, are actually reporting ventilation issues unless excessively trimmed in.

Perhaps even the Suzuki guys are running trimmed in....that would be interesting to know?

[DumperDriver]

Perhaps even the Suzuki guys are running trimmed in....that would be interesting to know?

I don't believe I've ever experienced any excessive ventilation/cavitation as described so often here, but maybe I'm not experienced enough to know it when it happens. ??

It was a stroke of pure luck (and a lengthy story which I won't bore you with) that I've ended up with a Suzuki DF20 EFI on my T38 as I didn't buy them together or plan it at all,... but all I do know is that it seems a perfect match and goes like stink !

I haven't added anything to the transom height and I have the trim pin one hole away from being completely vertical if that makes sense. Not sure if this means its trimmed in or out

[harrison]

I have the trim pin one hole away from being completely vertical if that makes sense. Not sure if this means its trimmed in or out

Thanks for the info. Going to assume your running one hole out from nearest the transom then. I’d call that position 2 and also say that’s trimmed quite far in...all depends on transom angle though.

Great to hear your rig performs well.

To counteract the ventilation issue we have to trim in/down to position 2. The pics below indicate what this looks like;

Trimmed in Position 2 of 6

I think this pic graphically indicates how far the motor is actually trimmed in, and how wrong it is. Any hull requiring this much trim-in to counteract ventilation has some serious issues.

[Bigplumbs]

Most of the outboards I run that are fixed are in position 2

[warder54]

On my old Honwave 3.8 ie I also ran with trim in 2nd hole out and it was perfect, both with a Mariner 15 2 stroke and my old Parsun 9.8 4 stroke. no issues at all unless the floor was under inflated. Has something not obvious changed in the floor profile of recent Honwaves?

[Bigplumbs]

Went out for a test today in the T38 with my new to me 2016 20 hp Mercury 4 stroke:

Trim position is in hole 2 and the motor looks vertical and as it should be
Top of transom to cav plate is 17 inches
2 fat blokes in it and some gear we got 18.5 knots and with just me in Just about 20 knots.
Not a hint of cavitation