Mariner 15 efi and Honwave T 32 I.e

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I've just bought a new 15 efi Mariner and it has a 9" prop on it. Hole shot is amazing and beat my friends 15 yam 2 stroke hands down.

Top speed is less though so I think I may be under propped for the load I carry.

Has anyone played with props with this set up please and can offer advice?

I was thinking a 10" pitch may be the right compromise.

[harrison]

To be honest the only sure way to ascertain if you’re under/over propped is to fit a tacho and see how many revs you’re pulling at wot.

However, we ran a 10” prop on our Mariner 20/T38 combo and felt it was spot on for our use. Very slightly under propped one up, it was able to easily bounce of the rev limiter, but with the benefit of great hole shot.

The 15 isn’t far behind the 20 as, being the same cc develops similar torque, so lightly loaded may well be able to pull a 10” pitch prop. Generally each one inch increase of pitch will reduce revs by around 200 rpm.

As said though, the only way to really tell is to get a tach and go play.

[BoatyMcRibFace]

@Harrison- I have a t38. Ran a 9.9hp yamaha 4 stroke last season but under powered. Looking at buying a new mariner 20. I felt the short shaft just never seemed to quite 'feel' right. I'veread about a wake bubble being created and causing prop issues. So I was going to get a long shaft and clamp it a bit higher on the transom. What shafy do you have and how do you find it? Thanks

[ChrisParish]

The T38 hull is prone to causing problems when it comes to ventilation. Generally the solution is to trim the outboard right in (hole nearest the transom).
Don’t get a long shaft - that’s a really bad plan.

[Zoom]

To be honest the only sure way to ascertain if you’re under/over propped is to fit a tacho and see how many revs you’re pulling at wot.

However, we ran a 10” prop on our Mariner 20/T38 combo and felt it was spot on for our use. Very slightly under propped one up, it was able to easily bounce of the rev limiter, but with the benefit of great hole shot.

The 15 isn’t far behind the 20 as, being the same cc develops similar torque, so lightly loaded may well be able to pull a 10” pitch prop. Generally each one inch increase of pitch will reduce revs by around 200 rpm.

As said though, the only way to really tell is to get a tach and go play.

Thanks for your comments.

I have bought a clip on tacho and when covid allows I'll do some testing.

As you say the 15 and 20 hp versions have the same cc so torque is similar.

I once had a Merc 75 4 stroke (Yamaha head) and that was the same cc as the 115 version.

It pushed my Fletcher 19 Sports Cruiser along 2 up at 27 knots which was impressive.

It's all down to the cc in the end.....

I'll update this post when testing complete.

[BoatyMcRibFace]

The T38 hull is prone to causing problems when it comes to ventilation. Generally the solution is to trim the outboard right in (hole nearest the transom).
Don’t get a long shaft - that’s a really bad plan.

I have spoken with several owners and a dealer or two who all do this or sell to people who do it.
I would rather have a short for.boot space!

[harrison]

Looking at buying a new mariner 20.

So I was going to get a long shaft and clamp it a bit higher on the transom.

As @ChrisParish stated...deffo don’t go long shaft. That’ll cause a whole host of far more serious issues.

On the T38 trimming in will usually alleviate the annoying ventilation issues as will ensuring the floor is kept at absolute maximum pressure. Our T38/Mariner 20 combo would run most days quite happily trimmed out on pin three. Other times, when ventilation issues inexplicably occurred, then trimming in one notch would cure the problem.

Just an observation, in my experience the Merc/Mariner 20’s seem slightly more intolerant of the T38’s ventilation foibles than the Suzuki 20. The lower leg profile, large AV plate shape and 1/2” or so extra leg length of the Suzi just seem better suited to the T38. The Suzuki is also considerably lighter despite the small differences in quoted weights.

[BoatyMcRibFace]

Looking at buying a new mariner 20.

So I was going to get a long shaft and clamp it a bit higher on the transom.

As @ChrisParish stated...deffo don’t go long shaft. That’ll cause a whole host of far more serious issues.

On the T38 trimming in will usually alleviate the annoying ventilation issues as will ensuring the floor is kept at absolute maximum pressure. Our T38/Mariner 20 combo would run most days quite happily trimmed out on pin three. Other times, when ventilation issues inexplicably occurred, then trimming in one notch would cure the problem.

Just an observation, in my experience the Merc/Mariner 20’s seem slightly more intolerant of the T38’s ventilation foibles than the Suzuki 20. The lower leg profile, large AV plate shape and 1/2” or so extra leg length of the Suzi just seem better suited to the T38. The Suzuki is also considerably lighter despite the small differences in quoted weights.

Ok thanks. Unfortunately I am tied to a mariner due to friend of a friend staff discount ! Know nothing about Suzuki and never considered one. Have been looking for 2nd hand tohatsu 20hp up til now. Love the t38 but got serious queries about it's short shaft performance. New 20hp will be the ultimate test

[harrison]

I have spoken with several owners and a dealer or two who all do this or sell to people who do it.
I would rather have a short for.boot space!

Am I reading this right?

You’re saying some dealers are actually recommending running long shaft motors on the T38.