Volvo diesel/skipjack prop advice and pyrometer advice needed

neversink2 · Aug-30-2005, 08:46 AM

I have a set of A4 props (duoprop) on my skippy - AD41, 200hp. The boat does 24.1 kts cruise at 3600rpm and 26.5kts at 4000rpm WOT. Is the engine hitting a rev limiter at WOT? I'm wondering if I have room to re-prop with A5's (I think) and get higher cruising speed while running the engine at lower rpm. The boat gets excellent mileage at 3600rpm (3.1 to 3.3mpg). I guess my question is - am I underpropped and am I overdriving the boat at 3600rpm cruise. The engine is rated at 3800 max rpm give or take a few.

Also, I think it would be good to add a pyrometer but don't know what exhaust gas temp is safe. Again, 3600rpm may be overdriving the boat at 3600rpm.

Can anyone help with this? Thanks...

FishWiz · Aug-30-2005, 09:53 AM

Chris, The variable you are leaving out of this equation is engine temperature. What temperature are you running at WOT? If it's still below comfortable operating range, then you are underproped. If it is within operating range, or you can actually push it hard enough to see the temp creap up a little beyone what you see at cruise (3600 RPM), then you are propped correctly.

ALSO, all tests need to be done with full fuel, all passengers, bait tank full, full gear, and about 5 cases of beer (DOH, that's my overnight trip). Anyhow, you get the point.

I have a KAD42 (230HP), on a 26 Blackman. I run A4s on 1:78 gearing. I can do about 24 knots max WOT, but I do less when fully loaded and don't run over 3600RPM.

Papa "J" · Aug-30-2005, 10:37 AM

For Pyro advise call the guys at SD Speedo Tach. 619-234-0315 Ask for Nacho. Very important that you measure as close as possible (within 6" to 12") for the run of the cable. Pyros are set up with that in mind and are basicly made to order for each specific boat.

We will be measuring my run some time this week & will be ordering 2 double pyro set up since I have twin turbos. I was told around 750 deg was a good temp for me to run but we have totally different applications.

Double Z · Aug-31-2005, 12:05 AM

This is based on your numbers. If your manual says the max RPM (redline) is 3800, don't exceed it. Are you quickly hitting the rev limiter or does it take a while to get up there? Can you spin through the props if you give it too much throttle too quickly? I demo'd a Fountain that did that with a 300hp Yanmar and a single prop. That motor pushed right past the prop and cavitated.
At what rpm does your motor make peak torque and what rpm does it make peak HP?

How does the motor "feel" at lower rpms? If you start moving, and get to the point just before coming up on plane, what happens when you give it more throttle? How about doing the same thing while loaded with full fuel and gear, going up the back of a large swell? Putting in a pyro would be a great place to start with finding out if your motor is happy or not.

getagaff · Sep-02-2005, 10:38 PM

Have a prop shop pitch you up slightly and tune the props. The A5's will drop you apprx 300-400 rpm which is to low. Your in the right ball park for what you want to accomplish but the jump in pitch on the stock set is just a little to much and will over prop you. Your looking to drop 200 rpm to achieve your 3800 wot but as fishwiz pointed out you should be able to achieve this rpm on a full load.

BiggestT · Sep-02-2005, 11:29 PM

For recreational applications, Volvo recommends 3800/3900 rpm at WOT, and max cruise speed at 90% of WOT. I think cruising at 3600 RPM is not wise on your boat. As others said, you also need to pay heed to the operating temp. Best measure is a pyro before the turbo.

neversink2 · Sep-03-2005, 01:54 PM

Thanks for the great advice guys - I think a minor pitch adjust sounds in the cards. My wife was watching me back the boat up last week and said "stop" right after I hit a palm tree with the prop. Result - 1 bent prop blade! My bad since I expected her to say "stop" BEFORE I hit the Fucking tree!!!

I'm gonna get that pyro too... just didn't know what exhaust temps were safe.

Thanks again guys,

Double Z · Sep-03-2005, 07:23 PM

As far as temps go, you should contact the maker. Find out what's peak sustained and what's peak for a temporary hit like just getting the boat out of the hole. Also, make sure they tell you if the temps they are giving you are pre or post turbo. Many people say pre turbo is the only way to go. Post has some advantages but is lower and reacts slower to change. Both is the ultimate but is pricey due to two pyro's and a combo gauge. I don't want to say what I have my trucks defueling set at, that always starts an arguement.

stp · Sep-03-2005, 11:14 PM

NAte you homo.........what are you doing...