Is my dingy inadequate?

ashedd

Petty Officer 2nd Class
Joined
Jun 16, 2015
Messages
175
So exactly how much speed should I expect from my 14' SIB with an older 20hp johnson? The best I can do is 19 mph(via gps) by myself.

The motor is old but runs great, it will start cold on the first pull. After that it will start on a half pull all day long. I put a Davis Doel-fin on it and it comes on plane quick as poop, before the foil the bow would point to the sky before coming down, and it would porpoise pretty bad. I also had cavitation/ventilation issues, and the foil has fixed all that.. My 9X9 prop gives it great hole shots, you better hold on tight! I have a 9X10.5 that I tried, and I went down to 17 mph with noticeable lower engine rpms, I don't have a tiny tach. I haven't measured the distance between my transom bottom to cavitation plate with a tape measure, but it's a "fist" in distance, and I have big hands. I'm getting a lot of splashing on the transom after the foil install, enough to occasionally get wet. I did notice some pressure loss in the tubes that I topped off after in the water. My keel on the other hand seems to not stay at 5 psi, it goes down to 2-3 psi. I topped it off before my speed runs.

My SIB is a real cheap chineese 14' 1.2mm PVC boat with aluminum floors. It's the same as the "stryker" boats from Canada. My boat actually came with a Stryker manual in the box, so it's from the same manufacture as my Aleko.

Any ideas? I was expecting at least 25 mph lightly loaded with a 2 cycle 20 hp tiller outboard. I'm on the lowest trim peg and the motor looks like it's at a 90 degree angle to the tubes.
 
Last edited:

fbpooler

Petty Officer 1st Class
Joined
Aug 13, 2011
Messages
334
I doubt you would ever see 25 MPH out of that rig even with the Doel fin drag removed and everything balanced out properly. SIB's have a lot of drag as compared to hard hulls. Your speed seems about right as it stands.
 

Chopperbill

Petty Officer 1st Class
Joined
Mar 26, 2014
Messages
389
Not sure of your post title, thought it might be a little personal. LOL
With my signature the fastest I have been is 25 solo. Most of the time I can get about 23. This is at sea level. But this ole fart doesn't really feel that comfortable above 18-20. It rides and handles better at that speed. Plus I don't like running near the motors rpm limiter. I have an air floor but I keep it and the keel HARD, as I was told buy the distributor. 5# on the keel is not enough. Boat is worthless with a soft floor and keel. Top speed in a SIB is kinda of a man thing, not truly as important as it sounds. Ease of planing and and how slow it will stay on plane is more important to me. That's why I leave the Stingray on.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Some issues :

What that speed achieved with you alone ? Need to trim engine to achieve sib riding parallel to water level, provided that sib is well weight deck balanced. While tiller driving, can sit a mate up front to balance sib evenly. If you experience cavitation/aeration prop issues on straight calm water courses, on slight choppy waters, and worst close tight turns at speed need to lower engine untill all those flaws dissapears.

On the other hand, if wetting yourself when doel fin is on, need to raise engine a bit, can add 1/2 to 1" wooden shim under engine and check if water splash has gone forever. If you trim engine +,- from ideal 90 deg angle with respect to sib's bottom, will lose best prop thrust and slight top end speed.

You need to install an induction tach to check wot revs as playing with different props won't tell much by ear taching, you could be over revving or even lugging the engine without knowing about it which is no bueno for the engine.

If engine and prop are 100% healthy, engine sits well trimmed at its best transom height, deck weight is evenly balanced, tubes and keel ar top inflated to its factory working stated PSI once sib rests on water, could better that achieved 19 MPH. Can go for a prop maximization to fine tune that combo to fully suit your boating needs.

No need to inflate keel above 4 PSI, you will end bending or even breaking bow wooden board in the short run. Air deck with inflatable keel is a different story.

Happy Boating
 
Last edited:

ashedd

Petty Officer 2nd Class
Joined
Jun 16, 2015
Messages
175
Some issues :

What that speed achieved with you alone ? Need to trim engine to achieve sib riding parallel to water level, provided that sib is well weight deck balanced. While tiller driving, can sit a mate up front to balance sib evenly. If you experience cavitation/aeration prop issues on straight calm water courses, on slight choppy waters, and worst close tight turns at speed need to lower engine untill all those flaws dissapears.
Yes that speed was alone with about 4 gallons of gas(tank at transom), about a 15 lb bag-o-crap up front, and a 5 lb tool bag for emergencies. Before the fin I got cavitation/aeration in calm water WOT(depending on weight distribution), in all turns and in any remote chop at all. I have to go through two canals on my way to open water and the chop can get really bad with the other boats, I would have to constantly slow down to idle and start again to get the prop to bite. With the fin all that is gone. So I originally figured the engine was too high, but like I said, the fin fixed all that. So perhaps the engine is now too low? The cavitation plate is about three inches from the bottom of the hull. The motor looks pretty parallel as it sits in the last "down" peg currently. I tried to go up one peg with the fin and got mild cavitation at times, so I guess I'm where I need to be on the bottom peg, no more holes to go down further. I did try no peg once, but it didn't change anything. Its nice to be able to get on plane and throttle back to a slow no wake(ish) speed for the channels
 

ashedd

Petty Officer 2nd Class
Joined
Jun 16, 2015
Messages
175
Oh, and my prop was slipping as has since been redone at prop shop a few weeks ago. These speeds were taken yesterday on lake Washington.
 

ashedd

Petty Officer 2nd Class
Joined
Jun 16, 2015
Messages
175
I doubt you would ever see 25 MPH out of that rig even with the Doel fin drag removed and everything balanced out properly. SIB's have a lot of drag as compared to hard hulls. Your speed seems about right as it stands.

My next one will be an actual RIB, there are a lot of large-ish ones on CL that need a little TLC. Two for around $400! Just need a place to put a real trailer, not my flat folding harbor freight special. I currently keep the rig all setup on the trailer at a boat yard for $100/month, literally 1 mile from boat launch, five from home, and plan to break it all down at the end of the season and lug it home. If I could justify with the boss to keep that yard spot all year round, then I get my RIB. As it stands my expenses will be looked at closely at the end of the year to justify ownership. I keep saying that the cheapest boat rental around here is $500 a day and the smallest is a 19' ski boat, I use my boat twice a week at least. :rolleyes:
But what do "I" know, we'll see at the end of the year.
 

7times6

Recruit
Joined
Aug 4, 2015
Messages
1
So exactly how much speed should I expect from my 14' SIB with an older 20hp johnson? The best I can do is 19 mph(via gps) by myself.

My 9X9 prop gives it great hole shots, you better hold on tight! I have a 9X10.5 that I tried, and I went down to 17 mph with noticeable lower engine rpms, I don't have a tiny tach.
.

You're about right in line with my setup which is very similar with the same results. Mine is a 15' SIB 1.2mm PVC with a newer 20hp all manual Merc. Put on a 9.25x9 prop and a hydrofoil. With my family of 5 our total weight with supplies, fuel, boat & engine is 1,050lbs, our top speed is around 18/19 gps. Just recently with another adult and a young adolescent, we hit 20mph gps.

Before, we had the same 9.25x10 standard prop that comes with the engine and would only hit 16/17.

Currently working on engine height. With no mods, our cavitation plate sat 1.5" below the transom/hull. Recently tried 1/2" shim which didn't affect the speed but helped with the splashing. Going to hopefully try higher next time to see what the results are but don't expect much of anything.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
You should not play with prop pitches if a induction tach is not installed, if you go wot with a niner pitch all alone while lightly loaded will surely be over revving engine badly. You can even pull a rod out specially if engine does not have a rev limiter. If sib will be family oriented for constant 5 boaters use, dial a pitch that revs at least middle wot rpm range, if near fully much better.

It's not a matter on how much top speed you can gain at wot with a given prop, it's a matter for engine to rev inside safe factory min-max wot range while moving load.

Happy Boating
 
Top