RPM vs. MPH Check In

[Chris Hyde]

Does 40mph top speed and 3400rpms sound about right for the following combo?

• 1992 Wellcraft 186 Eclipse XL
• 250lb driver/150lb passenger
• 100lbs fuel
• Volvo 431 (4.3)
• Volvo DP-C outdrive
• Aluminum B7 duoprops

In my opinion, the top speed looks ok, but it seems like this engine should spin more than 3400rpms.

Opinions?

 

[Don S]

Re: RPM vs. MPH Check In

Check your tach before you do anything else.

 

[achris]

Re: RPM vs. MPH Check In

3400 RPM at Wide Open Throttle is WAY too low. You need that engine to be spinning at 4400-4800 at WOT, or you'll kill it.

A 4.3 in a 18 1/2 footer should be doing a lot more than 40mph. I would expect at least 15 more. I have a 4.3 in a 20 footer (and a heavy boat at that) and I get 50mph.

Other info that would be useful here for my prop program are: prop pitch, prop diameter and outdrive gear ratio.

Cheers,

Chris............

 

[Chris Hyde]

Re: RPM vs. MPH Check In

3400 RPM at Wide Open Throttle is WAY too low. You need that engine to be spinning at 4400-4800 at WOT, or you'll kill it.

A 4.3 in a 18 1/2 footer should be doing a lot more than 40mph. I would expect at least 15 more. I have a 4.3 in a 20 footer (and a heavy boat at that) and I get 50mph.

Other info that would be useful here for my prop program are: prop pitch, prop diameter and outdrive gear ratio.

Cheers,

Chris............

Not sure on the pitch because I am not sure of a way to convert Volvo's DP's to pitch. The prop is a B7 and the gear ratio is stock as far as the previous owner was concern, which I am not sure, but the manual says:

• 1.78:1
• 1.95:1
• 2.30:1

Would my RPM issue be prop related or engine related? I wonder if the throttle is simply not adjusted to go far enough on the carb linkage.

 

[HT32BSX115]

Re: RPM vs. MPH Check In

Chris,

if that propset is the correct one and previously produced RPM in the correct range, then it's an engine problem.

You MUST get a reference tach and check yours and use GPS or other speed measure to ensure that you're starting with accurate data.

Cheers,

Rick

 

[Don S]

Re: RPM vs. MPH Check In

You should have a 2.30:1 gear ratio for that engine. The pitch and/or diameter of the props are not the same for each, and the props are setup as sets. DO NOT mix inner and out prop sizes, it doesn't work.
While the B7's are one of the steepest set of pitches you can get, you would have to drop down to a B2 and with luck you might get in range. That means something else is wrong. As I said before, check your tach first. Then look for other problems. 40mph might mean nothing if you are using the pitot tube type speedometer. They are really only a rough guesstimate of the actual speed you are going.
You have a lot of things to check besides props.

List of possible causes of low WOT.

Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[Chris Hyde]

Re: RPM vs. MPH Check In

Yes, I have a matched set of B7's on it. It runs great, but it reaches 40mph and 3400rpms. I will check the tach and check the speed with a GPS.

 

[mtnrat]

Re: RPM vs. MPH Check In

Yep as Don says, numbers mean nothing off of boat gauges if they have not been verified. I like to use a timing light with a tach. I connect the timing light and let it sit outside the engine bay and have a passenger tell me the RPM at different GPS speeds. My tach is pretty close, but my speedo is way slow. Beg borrow or steal the needed tools.

 

[HT32BSX115]

Re: RPM vs. MPH Check In

Chris,

Did you buy the boat that with those props on it and has it always performed that way? It seems unlikely that it would perform correctly with the highest pitch props available on a V-6.

It's appearing that someone may have put "taller" props on it thinking they could get more speed out of the top end.

Of course, It just doesn't work that way. Assuming the engine is running good and tuned correctly, you "prop" it for max recommended RPM at WOT and accept what ever speed you get.

 

[Chris Hyde]

Re: RPM vs. MPH Check In

Yes, those props were on the boat 4 weeks ago when I bought it. The engine is new and runs very well (other than a slight hesitation in the transition from idle to full throttle). I have a timing light, but I am going to see if I cant borrow another tach and a hand held GPS. I have 285 guys in my 4WD club and about 5 of them have portable GPS units.

 

[Don S]

Re: RPM vs. MPH Check In

If you find that the engine and drive is mechanically working properly, don't forget that there is a Prop Forum here at iboats, specifically for prop problems and questions.

 

[HT32BSX115]

Re: RPM vs. MPH Check In

Chris,


If you had "normal" (to me) props I would say plug it into a prop slip calculator and figure out what you "should" have on that sterndrive.



I did a little searching around and found a Volvo Penta publication that might answer some questions...... and while it's really a subject for the "Prop" forum, You'll get some pretty good info here and if you have more prop questions you can ask them with some additional info under your belt!

It appears that since your drive is a 2.30:1 ratio, A B7 propset might be the correct ones to have. AND judging from your speed (if it's accurate) the engine might be actually turning the correct RPM and you just have a simple tachometer problem.

According to the chart for the B series props a 4.3L engine with a 2.30:1 drive will give you somewhere around 40 mph at WOT.

If the engine really was turning only 3400 rpm you wouldn't be going 40 mph.......

Sounds a lot like the tach is the culprit (if the speed is anywhere near accurate)


Click HERE for the info. It's a PDF so you'll need the (free) Adobe Acrobat Reader (or equivalent) to open it....Most have it....If you don't, go here for it


Cheers,


Rick

 

[Chris Hyde]

Re: RPM vs. MPH Check In

Rick, I REALLY appreciate you taking the time to help me research this. I know you didn't have to. This is what sets people like you out. If I can ever help you with side graphics, boat wraps, hull numbers, etc. don't hesitate to let me know. You have a favor in the cue with me.

Here's a quick before and after of my vessel.

The 17 year old worn out graphics the day I bought it:

Yours truly at the helm yesterday, entertaining some family friends:

 

[HT32BSX115]

Re: RPM vs. MPH Check In

Hey the grahpics look pretty nice!! Sharp boat. All those Wellcrafts are cool looking.

Now all you have to do is pull out all the Volvo stuff and put a real stern drive in it!!

JUST KIDDING!!



Cheers,

Rick

 

[Chris Hyde]

Re: RPM vs. MPH Check In

HAHA! Thanks!

 

[mtnrat]

Re: RPM vs. MPH Check In

I have an 18' boat with a 383 with 350+ hp. I run a C6 propset and get about 38mph at 3400 rpm. I am very interested what your true mph and rpm are. I think the B7 may have a little more pitch than the C6 but the B7 is aluminum and the C6 is stainless steel, so has much less flex.