Twin Suzuki 20s on a 16 ft Sylvan

[Flygaff]

This is what I'm doing to my lake boat. I have a 1993 Sylvan Pro Select SC boat. Original owner. Purchased new in 95. I am replacing the 1995 Evinrude 9.9 with twin Suzuki 9.9 converted to 20s. The local lakes have a 9.9 HP restriction. They don't seem to mind how many of them you have on the back. This is what I have done and found so far. In the spring I mounted a single 2021 Suzuki 9.9 long shaft elec start w power trim. On the first test I was only getting 7.5 MPH and 4200 RPM. I then converted the engine to a 20 HP by changing the ECU and inlet plate. after that I was able to get 9.4 MPH at 4700 RPM. Changed the prop a few times and it made little change. I then added the 2nd 2021 Suzuki 9.9 converted to 20 HP and changed both engines to 9 1/4 x 11. First test I was able to get 18 MPH at 5100 RPM. Changed the props to a 10 inch pitch. Big difference. 25 MPH at 5700 RPM. Getting there I think. My problem now is I get a large rooster tail shooting water everywhere. I believe my cavitation plate is too low and needs to be raised. I just received two TH Marine Mini jack plates. The Mini plate will give me a 4 inch lift and 4 inch further back off the transom. Now I have a 22 inch center between the engines. I plan on moving them to a 17 1/2 center for ease of mounting the jack plates. My question is What do you think moving them closer will do? Higher RPM?, Cavitation for the props being closer together? My goal is to get the engines running at 6200 RPM and I hope to get the speed to near 30. What do you guys think.

 

[gogittum]

C

 

[gogittum]

Couldn't even guess, but it sounds like you're having fun. Sounds like you were over-propped with the single engine. Some will make negative comments and tell you to just put a 30 or 40 on it,, but it's a 9.9 lake and it's your project. Have at 'er and keep us posted.

40 some years ago I had a '60 Ford F250 4WD pickup that I'd put a 352 T'Bird engine in. The body was badly rusted. I lived in a very small town in north Idaho at the time. A beat up old '62 Ford Econoline van with the little 232 c.i. 6 cyl engine came available for next to nothing and I grabbed it.

I spent a few months building a sub-frame under that uni-body van, then moved the 4WD and 352 under the little van, complete with 7.50/16 tires. It stood waaay up there and looked kinda odd....kinda neat. The '62 was just a little van. People all over town shook their heads, wondering WTF I was thinking about.

It was a good experience, a challenging project and a lot of fun. I came out of it with a totally unique toy that turned heads every where I went and the thing would nearly climb walls. Go for it and have fun.....more fun than you already are.

 

[Pappy]

Moving the engines together more on this set up will probably work well. The rooster tails you mentioned are nicknamed angel wings and are indeed an indication of engines running too deep below the boat bottom. Good call on your part. They should not ventilate more at all unless there is an obstruction on the boat bottom ahead of either engine. (through-hull or something else).

 

[Flygaff]

Moving the engines together more on this set up will probably work well. The rooster tails you mentioned are nicknamed angel wings and are indeed an indication of engines running too deep below the boat bottom. Good call on your part. They should not ventilate more at all unless there is an obstruction on the boat bottom ahead of either engine. (through-hull or something else).

Thanks for the comments. As soon as the snow melts I will mount the Jack Plates and remount the engines on 18 centers. I will post the results when available

 

[Jim]

Let’s go! Looking forward to seeing this come to fruition!


Sent from my iPhone using Tapatalk

 

[Ronbedard57]

Seriously!? 9.9 restriction but they don't care how many you mount on the transom? I think our Marine Patrol officers would laugh while writing the ticket.
Sounds cool! I've got a pretty nice old 28 Johnson, and could probably find another one. I think they'd outweigh the 40HP I'm running by too much though.
Good luck!

 

[DaleH]

FYI, on all the bodies of water I know that are like that - with OB motor HP restrictions - if 2 OBs, they make you take off one of the props and leave that 2nd OB tilted up & out of the water ...

Sounds like a cool, albeit expen$ive, project!

 

[MN Fisher]

FYI, on all the bodies of water I know that are like that - with OB motor HP restrictions - if 2 OBs, they make you take off one of the props and leave that 2nd OB tilted up & out of the water ...

Sounds like a cool, albeit expen$ive, project!

Ya - Minnesota is the same way. If they say 10hp limit...they mean total hp. If they suspect you've 'souped up' your motor...they'll test it...and fine you if it produces more than what's labeled.

 

[Flygaff]

I ran twin 15 HP Evinrudes for 20 years without a problem. I'm on the water early and off the water before most people show up. A little common sense around the boat dock also helps.

 

[Crazyboat]

This is what I'm doing to my lake boat. I have a 1993 Sylvan Pro Select SC boat. Original owner. Purchased new in 95. I am replacing the 1995 Evinrude 9.9 with twin Suzuki 9.9 converted to 20s. The local lakes have a 9.9 HP restriction. They don't seem to mind how many of them you have on the back. This is what I have done and found so far. In the spring I mounted a single 2021 Suzuki 9.9 long shaft elec start w power trim. On the first test I was only getting 7.5 MPH and 4200 RPM. I then converted the engine to a 20 HP by changing the ECU and inlet plate. after that I was able to get 9.4 MPH at 4700 RPM. Changed the prop a few times and it made little change. I then added the 2nd 2021 Suzuki 9.9 converted to 20 HP and changed both engines to 9 1/4 x 11. First test I was able to get 18 MPH at 5100 RPM. Changed the props to a 10 inch pitch. Big difference. 25 MPH at 5700 RPM. Getting there I think. My problem now is I get a large rooster tail shooting water everywhere. I believe my cavitation plate is too low and needs to be raised. I just received two TH Marine Mini jack plates. The Mini plate will give me a 4 inch lift and 4 inch further back off the transom. Now I have a 22 inch center between the engines. I plan on moving them to a 17 1/2 center for ease of mounting the jack plates. My question is What do you think moving them closer will do? Higher RPM?, Cavitation for the props being closer together? My goal is to get the engines running at 6200 RPM and I hope to get the speed to near 30. What do you guys think.

This may be way too late of an answer but if your transom is designed for a short shaft you are definitely drawing too much water and that can be dangerous when steering. I'd build up the transom height by 5" and leave it at that. If you have the jack plates I'm sure you'll see a difference and it may be right as it is now set back from the transom a few inches as well. IDK how big your boat is but with 40HP I'd think you'd get 25 MPH min with that set up.

I do find it hard to believe that no one seems to care about the number of outboards you've got hooked, but hey, if you can get away with it, go. What is the max HP rating of your boat, maybe go for triplets?

 

[airshot]

You will never see 30 mph unless the hull is bare bones..two motors are generally not as fast as a single engine of the same hp. You may wind up over reving your engines, then you will need oars...I have a 16' Sylvan SC with a 40 hp Merc two stroke and can just get 31 mph with optimum trim and one person..

 

[Flygaff]

Well, that project is complete. I ended up maxing out at 28.72 max speed. It worked great after some tweaking. I have sold that hull and mounted the twins on my new to me 2010 G3 Angler 185F. The top speed on this is 11.4 mph. I would have been happy with 16 but too heavy. The hull is 1700 lbs. so I sold one of the engines and am in the process of hanging a Suzuki 140

 

[Flygaff]

I did think of running triple motors. I have since regained my sanity. There was a guy a while back that I have seen on the lake with triple 9.9 Yamis on an 18 ft center