Top end speed...

[Bob M]

After purchasing my 320, I drove it back seven hours from Eastport LINY. During the journey, at times I would be doing 23-24 knots over the bottom at roughly 3800 rpm's. Now I have to do 4000-4200 rpm's to go 20 knots. I do not like running my boat at such high rpm's. I had a diver check the bottom and he cleaned the running gear and told me the bottom looked okay. I've tried postioning the trim tabs from up to down and I can't attain the speed I would like at the alleged sweet are of 3600-3800. Am I doing something wrong, or do I need to just get used to traveling at higher rpm's? Gas tanks were 3/4 full during the last run. When I left LINY the tanks were full for that run.

 

[skolbe]

What is your WOT? Mine hits 5000 RPM's.

 

[Bob M]

I have the 350/300HP Horizons. The most I've pushed them so far is around 4300 rpm's. I guess I haven't totally forgotten the performance of my 30' Weekender with twin 260HPs, and straight inboards. That would plane out fast and cruise 20 knots at around 2800-3000 rpm's. I can see this isn't going to happen with my 320.

"It is what it is", but I'm trying to make sure what I perceive may be a problem really isn't one. Any feedback from fellow 320 Captains is appreciated.

 

[Hampton]

Your original numbers seem a little too good to be true, but the current ones are not very favorable. Is it possible that conditions other than the bottom fouling have changed? Did you fill it up with gas and water since then? Have you moved a significant amount of belongings on board? Dishes, towels, bedding, extra sodas, bottled water, a few cases of beer?

Adding extra weight, then going from a following wind to a head wind and possbily current too could significantly alter your performance.

 

[FSUDave]

I would say you have a problem somewhere. I cruise between 3600-3800 rpm which is between 24-26.knts by the gps. When my bottom is due for a cleaning the numbers drop but not like that.
Also unless you are familiar with your diver and you trust him I would get under the boat and check, I have seen some pretty horrific things from divers and their idea of a "clean" bottom.

 

[Defender]

Bob, as unattractive as this may sound, I would agree it may be a good idea to see how many RPM's you can get at WOT and if engines remain synced. If you had the boat surveyed at purchase the report would indicate RPM at WOT, check this against what you’re getting.

I am experiencing similar issue as I don’t feel the boat is running as strong at higher RPM’s this year even after diving and cleaning the props myself. I wonder if this is a possible fuel issue.

 

[Vince_nj1]

Your original numbers match my 320 cruise speed. (24.7 knots at 3950 RPMs) I had a similar experience when I put my boat back in the water this year. I had fresh bottom paint on the boat but it sat in the slip for about a month before I took it for ride. When I did I was struggling to get over 22 knots. The tanks were full and I had the eisenglass up. The next time I went out I took it for a long ride and burned up 70% of my fuel, by the end of the trip I was cruising back at my old speed.

For me, I chalked it up to a combination of things. Bottom paint probably had some growth that sloughed off with use, I may have been fighting a tide and the wind, and I had the eisenglas up and I had added weight on the boat with full water, fuel and holding tank. (At roughly 8lbs per gallon, that can add over 2000lbs to the boat.)

 

[barryg]

I just came back from Bimini running into head seas with around 2-3' rollers and we cruised around 3670- 3750 rpm and ran around 25-26.5 mph, with 4 aboard, almost full gas, and a dinghy on the swim platform.. You definitely have a problem and I would not go WOT untill you have the boat checked out.. 20 knots at 4000 is not right...If a diver can't find the problem-whether it's a fouled bottom or a damaged prop, you'll have to go further and possibly pull the boat out of the water to see for sure what is happening..

 

[2lkmead]

I have a 310 Dancer with 350 Mags and V-Drives. I too average 23-25 knots at 3600-3800 RPMs. Having said that, I believe I could do better if I removed even half of the gear on board. People at the Marina come to me as if I am a hardware store!

 

[Zorba]

NOt trying to be condesinding, I've done this myself, but by chance were you looking at MPH on your original trip and changed your settings to Knots on your GPS on your next trip?

At the risk of making myself look like an idiot, i'm sure someone else has done this before, we coulldn't figure out why my brothers Baja wouldn't do more than 50 MPH and it took two weeks to figure out that the GPS was set to knots.

 

[Bob M]

Thanks for all the replies! Lots of "food for thought" to be considered. My props were pulled before the boat went in the water. One would assume they were "tuned" properly. I filled the gas tanks before I left LINY. The water was also full. I haven't hit anything with the props since I've had the boat. I've only filled the gas tanks once since the trip back from LINY. I did add "Startron" to both tanks. I will fill the boat again this weekend. I don't feel any skipping or hesitation. It just feels like the boat is laboring to gain speed. Eisen glass is up with one panel in front of the helm removed. We have avoided adding a lot of gear at this point, but additional stuff will go on board for our vacation starting this Saturday. I'll make an attempt to check the bottom while I'm at Block Island. I will also check the mph versus knots. I'm pretty sure I was reading knots as SOG, but it doesn't hurt to confirm that.

Once again, thanks for all the input.