Upgrade motor or new boat?

babbot1 · 05 February 2021, 17:38

I have a Mercury 310 air floor with an old, very old 1982, Johnson 9.9. It still runs well and I have it propped with a 4 blade and lower pitch but we outgrew its performance. Family of 4, now almost all adults (except for my childlike mentality

) and a large dog.

Is it worth upgrading to the new Tohatsu 15hp efi or do I need a rib with a 20hp? The boat is in decent shape so don't really need a new boat. I would also hate to spend the $$ on the 15hp and find out it still doesn't plane. The max on my 310 is 15hp. And I can't find anything used that's worth looking at.

I imagine my 9.9 is probably more like 7.5 being so old so it will be an improvement but maybe not enough. It is pretty close to planning with this setup but just can't get over the hump and we used to plan about 3 years ago when the kids were smaller.

Anyone have a similar situation or try trim tabs with lower pitch 4 blade on the 15hp?

Pikey Dave · 05 February 2021, 20:41

Quote:

Originally Posted by babbot1

I have a Mercury 310 air floor with an old, very old 1982, Johnson 9.9. It still runs well and I have it propped with a 4 blade and lower pitch but we outgrew its performance. Family of 4, now almost all adults (except for my childlike mentality

) and a large dog.

Is it worth upgrading to the new Tohatsu 15hp efi or do I need a rib with a 20hp? The boat is in decent shape so don't really need a new boat. I would also hate to spend the $$ on the 15hp and find out it still doesn't plane. The max on my 310 is 15hp. And I can't find anything used that's worth looking at.

I imagine my 9.9 is probably more like 7.5 being so old so it will be an improvement but maybe not enough. It is pretty close to planning with this setup but just can't get over the hump and we used to plan about 3 years ago when the kids were smaller.

Anyone have a similar situation or try trim tabs with lower pitch 4 blade on the 15hp?

I’m not familiar with that engine, but is it part of a family? E.g. the Suzuki twin cylinder is available as a 9.9/15/20hp. Exactly the same engine/weight, just different mapping. If the Twatsoo was available in a 20hp, you could buy that, try it on your boat. If it doesn’t work out, you have an engine that you can put on another boat.

Limecc · 05 February 2021, 22:29

Try a fitting a Doel fin first, it will make a world of difference if you are borderline planing.

treerat · 06 February 2021, 07:28

Sounds more like it's the weight in the boat now the kids have grown. 4 adults in a boat that size will struggle to plane no matter what you out on the back.

Personally I'd be looking at something a bit bigger which would give you more room, more options and plane easily.

Fenlander · 06 February 2021, 11:50

>>>Personally I'd be looking at something a bit bigger which would give you more room, more options and plane easily.

100% agree. Even if you do manage to get it to plane it will feel vey cramped with 4 folks + big dog and (particularly being an air floor) in my opinion that's pushing it right to its limits of operating at speed.

We have run a a 3.8m for some years. Even that starts to feel full with 4 + dog and day out kit. Also when we went from using it 3 up to adding that 4th person I upgraded to a 20hp to get the response I felt I needed on and off the plane in more challenging sea conditions.

treerat · 06 February 2021, 12:55

Quote:

Originally Posted by Fenlander

>>>Personally I'd be looking at something a bit bigger which would give you more room, more options and plane easily.

We have run a a 3.8m for some years. Even that starts to feel full with 4 + dog and day out kit. Also when we went from using it 3 up to adding that 4th person I upgraded to a 20hp to get the response I felt I needed on and off the plane in more challenging sea conditions.

Used to love my 3.83 with a 25 on the back - used to be great with 3 on board but even that with 4 blokes struggled to get going.

Limecc · 06 February 2021, 14:35

Everyone's suggesting a bigger craft and of course this would need another engine.. Expense on expense. Also additional weight and handling effort.

But Babbot's happy with the present boat and also the performance if it would only plane. Grown up kids soon leave the nest or do their own thing..

As I said earlier, a Doel fin solves this the easiest and cheapest way. It will plane. Inability to get over the hump was the only complaint.

Escapeskiing · 06 February 2021, 16:49

I used to have a 3.1 airdeck with a modern 8hp four-stroke. That used to plane easily with 2 adults. I think a 15hp would make a huge difference. However, sure the consideration should be more about the boat size for 4 adults and a dog?? Ribs are all well and good but remember you have to trailer them everywhere and are that much heavier before you start adding people to the mix.

Steve509926 · 06 February 2021, 17:08

Can you borrow a 15hp to try on your Mercury 310?

I have an Excel SD360 with a Merc 15hp efi.
With 2 adults and a large dog (195KG) + fuel and kit, its great - planes quickly and is very responsive.
3 adults and a large dog (260KG) + fuel and kit, its good - planes ok and is still responsive.
4 adults and a large dog (350KG) + fuel and kit, its ok - slow to plane and often drops off, responds like a wet mattress, slow and heavy to steer.

If there was more than just the two of us and the dog on a regular basis I think I would have gone for a 20hp, as it is I am more than happy with a 15hp.

Another thing for you to consider is the weight of a 15/20 hp efi. I can just manage to lift my 15hp Merc out of the truck but I cant carry it very far.

Fenlander · 06 February 2021, 17:59

Steve a very useful realistic set of figure very much in line with our experiences.

treerat · 06 February 2021, 20:18

Quote:

Originally Posted by Limecc

But Babbot's happy with the present boat and also the performance if it would only plane.

As I said earlier, a Doel fin solves this the easiest and cheapest way. It will plane. Inability to get over the hump was the only complaint.

Guess you've not read the title of the thread? Clearly asks upgrade engine or new boat?

Fins wont help an underpowered, over loaded boat.

Limecc · 06 February 2021, 22:07

Quote:

Originally Posted by treerat

Guess you've not read the title of the thread? Clearly asks upgrade engine or new boat?

Fins wont help an underpowered, over loaded boat.

No I read the question, he assumes that's what's needed in the title.

It's a highly loaded small boat which will be pushing a wall of water trying to climb it at a steep angle. However he said it's on the verge of planing.

A Doel fin will give stern lift compensating for some of the weight and it won't have the same hump to get over. It will absolutely work, hideous looking that they are.

Lightly loaded it will plane earlier and at a lower speed, heavily loaded it will make the needed difference.

Also, I wonder if the Johnson would benefit from a decoke?

https://www.youtube.com/watch?v=ezNO...ature=youtu.be

babbot1 · 07 February 2021, 18:51

Thanks for all the responses. Lots to think about.
I don't like dolefins, I have had one on another boat and didn't like the handling.

The current boat is rated for 15. So a 20 is out for this boat.

Steve, those stats are great. I think I am going to try a 15 Suzuki first. If it doesn't work it seems the upgrade to 20 is not too bad and reasonable $$. The Tohatsu upgrade is not economial.
If the 15 doesn't work even with reprob and tabs I can then upgrade it to a 20 and buy another boat.

The size is acdeptabley. We fit, cramped but fit. We don't travel too far but it does take forever at hull speed.

Steve509926 · 07 February 2021, 19:15

Hope it all goes well with a 15hp Susi, do keep us all informed how it goes.
Where abouts are you in the States?

Pikey Dave · 07 February 2021, 19:40

Quote:

Originally Posted by babbot1

Thanks for all the responses. Lots to think about.
I don't like dolefins, I have had one on another boat and didn't like the handling.

The current boat is rated for 15. So a 20 is out for this boat.

Steve, those stats are great. I think I am going to try a 15 Suzuki first. If it doesn't work it seems the upgrade to 20 is not too bad and reasonable $$. The Tohatsu upgrade is not economial.
If the 15 doesn't work even with reprob and tabs I can then upgrade it to a 20 and buy another boat.

The size is acdeptabley. We fit, cramped but fit. We don't travel too far but it does take forever at hull speed.

Weight wise the 15-20hp Suzuki are identical. Same engine, just different mapping.

Steve509926 · 07 February 2021, 19:49

Quote:

Originally Posted by Pikey Dave

Weight wise the 15-20hp Suzuki are identical. Same engine, just different mapping.

Hi Dave, I think I get what you're saying - "why not go for a 20hp Susi as its the same weight"
That would make sense to me, but would that effect insurance?

Pikey Dave · 07 February 2021, 20:15

Quote:

Originally Posted by Steve509926

Hi Dave, I think I get what you're saying - "why not go for a 20hp Susi as its the same weight"

That would make sense to me, but would that effect insurance?

Depends on the insurance company, or if indeed the boat is insured.

Steve509926 · 07 February 2021, 20:21

Gives the OP food for thought!

Limecc · 07 February 2021, 21:01

Quote:

Originally Posted by Pikey Dave

Weight wise the 15-20hp Suzuki are identical. Same engine, just different mapping.

In addition to a different ecu, there's also a restrictor baffle in the inlet to remove and replace with a spark arrestor, otherwise the 9.9/15/20 are all the same engine.

babbot1 · 07 February 2021, 21:48

Quote:

Originally Posted by Steve509926

Hope it all goes well with a 15hp Susi, do keep us all informed how it goes.
Where abouts are you in the States?

Thanks. We are in Connecticut. Usually boat on the Connecticut river. It's a large river that ends in the ocean with lots to do.

I did look up the parts, I only found the scematic for a 2018 so not sure if 2021 is different. It's the same price for the 15 and upgrade parts as it is for the 20.

I don't want to put a 20 on my max 15 boat. Not sure what isurance would do if something happened but the authorizes do check around here. I go on the boat to ralax not to look over my shoulder. :-)