Speed Plateaus at 5200rpm

[jakec]

I just repowered with a new Suzuki 115. The dealer gave me a 21p 3 blade prop. The boat is a 1959 Dorsett San Juan.

I previously had a 1977 Mercury 115 and a safe/comfortable cruising speed of 29mph, and a top speed of 31mph (but with porpoising/reduced control). With the new engine the top/comfortable speed increased to 34mph. The speed plateaus at about 5200rpm (max 6200). Since the engine is under 10 hours I've only had the rpm up to about 5800rpm.

Regarding trim, the prop will ventilate before the boat begins to porpoise. The boat actually will not porpoise at all as a result. So when I'm running at 5200rpm+, with full trim, my speed is still pegged at 34mph and the boat is level.

I've posted on facebook and been told I may need to get the bow up, and a 4 blade prop was suggested. I don't NEED to go faster, but I would like to be able to use the full potential of the new engine. I also want to be able to perform with a load (4+ people and gear). I'm attaching a pic of my boat (red) to show the hull shape, which has a very deep bow, which I'm not sure is even possible to get out of the water? Other pic (blue) shows the same model on plane.

 

[QBhoy]

Hi. Any pictures of the engine and it mounted on the boat? It may be poorly set up for height perhaps. Could also be to do with the propeller too. Some props just reach their limit of grip and won’t take too much trim at all. You may be over propped too perhaps. Why was the decision to put a 21” on it made ? Was this some after testing for suitability or was it just thrown on at a guess ?

 

[Scott06]

What 3 blade 21" prop is on there now?

I had a big diff in top speed switching between a Vensura and Revolution 4 - both 4 blade 21"

The Rev 4 has so much better bite that I could fully trim up and gain 3-4 more mph top speed. Downside was that hole shot was better with the Vensura as the engine could spin up into the power range quicker...

 

[airshot]

If you have any prop shops in your area, go talk to them. A prop specialist can see and understand your issues and get you hooked up. Not only pitch, which most concern themselves with, there is also rake which plays a bigger role on getting the bow up. To get the max performance your looking for it just might take more of a custom designed prop than just another off the shelf prop.

 

[jakec]

Thanks everyone for the responses. Deep in winter now so I haven't thought about the boat in a while.

I do have a good prop shop near me. I will probably give them a call before the season gets started. I just wanted to get a basepoint from the forum so I don't end up buying 3 props from them trying to chase down the solution.

I got ahold of the salesman from my dealer and he did was not able to tell me anything about the prop other than that it is suzuki brand. And looking at the prop myself the only numbers on it are the pitch, which I already knew.

As for the engine mounting and prop selection, both done by the dealer and I have to assume the installation is correct. since the correct plate is aligned with the transom, it is centered, and I'm not getting any spray. The 21P was chosen because they did a sea trial with a 19 and said they could not get max RPM. Now you can get max RPM I just feel speed should be higher.

 

[racerone]

???-----They put a 21" on there because they could not get full RPM with a 19" pitch ?-----Ask them to explain this.

 

[Scott Danforth]

I read that as they had a 21p in stock and wanted to sell it

 

[Scott06]

Thanks everyone for the responses. Deep in winter now so I haven't thought about the boat in a while.

I do have a good prop shop near me. I will probably give them a call before the season gets started. I just wanted to get a basepoint from the forum so I don't end up buying 3 props from them trying to chase down the solution.

I got ahold of the salesman from my dealer and he did was not able to tell me anything about the prop other than that it is suzuki brand. And looking at the prop myself the only numbers on it are the pitch, which I already knew.

As for the engine mounting and prop selection, both done by the dealer and I have to assume the installation is correct. since the correct plate is aligned with the transom, it is centered, and I'm not getting any spray. The 21P was chosen because they did a sea trial with a 19 and said they could not get max RPM. Now you can get max RPM I just feel speed should be higher.

My local shop supplied a couple props to trial under you break em you bought em… ended up buying both because i liked one for skiing one for cruising. they should do the same otherwise you can buy them online for less…

 

[Texasmark]

Even though you have a nice wave slicing bow, your boat seems to have a "semi-V" hull characterized by that bow and a flat stern area. The flat stern is your best planing surface other than a "pad" type hull used on bass boats traveling at speeds exceeding 60 MPH driven by high HP engines.

I agree on the comment Scott made about the dealer had a prop he wanted to sell. You don't up the pitch to increase RPMs.....you do just the opposite.

On a 4 blade prop, Solace has SS 3 and 4 blade props. On their site, they have a tutorial about how both are designed and the end result desired. Some folks on here are die hard 3 bladers and others are more broad minded... taking the extra blade for certain performance features.....not taking sides either way.



I don't know Suzuki's lower unit gear ratio. Mercury uses 2.33 on 75 and 90 HP engines and 2.05 on 115 hp engines. Having had both engine ranges, the 115 engine is more prone to shallower pitch for a given load.

Speculating here, your Suzuki is also running 2:1 and a 21P prop on that boat, especially with 4 folks aboard is significantly too high.

Go-Fast.com > Knowledge base > Find your prop slip > Prop slip calculator

I find something very confusing here: Slip is running 35% and RPMs are 1000 Hz low. I'm going to say that your engine has crashed through its torque curve due to severe overloading caused by too much pitch in that prop.

Dropping to 17P and assuming that your RPMs will come on up to 6200 (which is a fair guess) since your engine will now be properly loaded, following it's normal HP-torque curves, and assuming that your slip will drop to 12%....another reasonable guess, your top end will run out at 44 mph...load in the boat at the time dependent. That hull of yours should support that speed very nicely.

Go-Fast.com > Knowledge base > Find your prop slip > Prop slip calculator

On 3 or 4 blades I would follow the Solace SS tutorial and go for 4 and they tell you why. I have one on my 115 (2:1 gearbox) and it does what's advertised on my Crestliner 1750 Fish Hawk. If you were running just yourself and maybe a light companion, then I'd opt for the 3. On comparing 3 to 4 blades, I also have run a 20P 3 blade laser...built for speed and as one would expect, the extra blade and inch less pitch cost me 3 MPH but most of my running these days is medium speeds and the 4 gives me better boat trim control....holds steady RPMs rather than run up...too fast, cut the throttle and falls back slowing, advance the throttle a tad and speeds back up etc. etc.

That's my thoughts knowing nothing more about your situation and using my boating experience as my rule of thumb.

 

[jakec]

I had to go back in my texts and I see that I was remembering incorrectly. So here is some additional info to my previous comment. This is what the dealer said to me after the sea trial:

"I tried a 19P and going to a 21. Got the boat to 34mph but I hit the rev limiter at 3/4 throttle."

Also, when I asked the same question I asked here in fewer words, his response was:

"It goes by hull speed, not horsepower, I think that's about all you're gona get out of that boat."

That said, I have talked to another owner of the same boat and model that I have and they are running a 2009 115hp Mercury, and said they reached 42mph. So I believe the hull speed comment is incorrect. 42mph is closer to the estimate Mark gave from the calculator website.

From the suzuki website, the gear ratio on my outboard is 2.59:1.

Putting in actual figures from my experience (21P, 31mph, 5200rpm) it gives me a prop slip of 22%.

Putting in the figures given to me from the dealer's sea trial (19P, 34mph), it gives an rpm of 4894, about 3/4 throttle.

Putting 18P and max rpm (6200) gives a speed of 41, close the anecdotal 42.

Putting info from the mercury owner, I get 16P.

 

[Texasmark]

Summarising your feelings with what has been said/seen ? Maybe your boat weighs a lot more than it did when manufactured......floatation foam has absorbed a lot of water, usually the case when the boat has been sitting in the water or outside uncovered.

With the 2.59 gear ratio on the Suzuki engine, I can see where the higher pitched prop is in order.....2.59/2 x 17 = 22 pitch right in line with the dealer's prop submittal of 21. One can only speculate that a new engine is performing up to it's design ratings so blame for poor performance lies elsewhere....speculation again.

Gut feel: With what you said about the new engine and gear ratio and the performance results so far, you have an underlying problem somewhere.......

 

[jakec]

Can you explain what you are doing here?

2.59/2 x 17 = 22 pitch

My boat certainly weighs more than when new, but that's because I did a full restoration of the stringers (no foam used), floor, and transom. So those items may have added a few hundred lbs. to the boat I'm sure. But the boat I'm comparing mine to is also a restoration and looks to be just as heavy as mine. It could be lighter, but I would think it would be in the tens of pounds.

The scale weight new is stated as 1500 lbs. When I've done speed calculators I always use a weight of 2000 lbs. I put the hull type variable at 170 which is in between their value for "average runabout" and "high speed runabout" to achieve 40.7mph.

 

[jakec]

Here are a couple more pics of my boat. The transom progress pic shows the hull shape well. The pic in water is just a nice pic

Another thought that crossed my mind is that the previous own had rolled on the paint on the underside of the hull, so there is a large area that is rough. Any area that was too horizontal to spray has a rough texture.

 

[Texasmark]

I think you found your "smoking gun".....figure of speech.....the rough hull area and it encompasses all the planing area when you are running at top end. To give you and example of how bad that can be, I used to take family vacations at the inlaws lake house and keep my boat in the water the whole time. It usually lasted 2 weeks.

I don't remember the numbers, but by the end of 2 weeks, just algae, marine growth or whatever you call it had significantly cut into my top end....and that's just slime, not a rough surface....was so bad that once I discovered the problem, every couple of days I would go out and scrub the bottom of the boat to remove it. The lake was down river from the Dallas-Ft. Worth area so you can guess as to the quality of the water........

Looking at the hull shape at the transom I see a perfect example of a fine planing "semi V" hull and these types of hulls require minimum power to push at a decent speed of all the hull types......so it isn't the hull, it's what's on it.

 

[jakec]

So do you think the prop I have is correct and I am really losing close to 20% of my top speed based on having a rough bottom?

 

[jakec]

Here is a pic of the texture.

 

[Scott Danforth]

A golf ball flies faster due to the dimples. Your texture is fine.

 

[Texasmark]

A golf ball flies faster due to the dimples. Your texture is fine.

Interesting comparison Scott. So tell me sir, just why do people who leave their boats in salt water use antifouling paint on the waterborne hull and remove their boat periodically for Barnacle removal?

I can't make heads or tails of the photo. Your hand can give you the answer....if it doesn't feel smooth it isn't and your hull isn't a golf ball in air.

 

[jakec]

I did some research and it seems that its not totally conclusive but they think that smooth is better, but it matters less and less the larger the boat and the faster you go. I wouldn't think it can account for the loss of 20-25% off of top speed.

 

[Scott Danforth]

Laminar vs turbulent boundary layer. Turbulent has less drag no matter if in water or air. Slightly bumpy will always be faster.

Anti fouling paint is to keep barnacle colonies from growing. Think oyster shoals growing off your hull.

The rough surface in the pic wouldn't lose 25% of top speed

A water soaked hull, wrong prop pitch, wrong rigging height, motor not up to snuff all would