Help: 320 with 5.0 VP sea trial having difficulties to be on plane at 3400 rpm

JLAS

Hey guys, 

A 320 with the bottom in poor condition how much would affect its performance?  I see on the forums that the 320's cruises at 3400 rpm at a speed of 25 mph or so.  This one needs to be at 3800 or 3900 just to be on plane and will get about 24 mph.    Lower rpm than that and the boat will stall.  Top speed at 4900 rpm will reach 34 mph. The bottom is in need to be painted, though.    Any ideas? Is the boat too far off? 

Thanks in advance. 

randy56

Don't know much about the 320, but I have to be at 3600 rpm to stay on plane. 

JLAS

Do you use the tabs at all?  I tend to believe the issue here is because of a dirty bottom, but not sure how much of a difference that can actually make. 

Liberty44140

Sounds ok. My 01 310 had twin 5.0. With 4 adults she really struggled to get on plane without tabs, even with two adults and a dirty bottom she struggled. But with tabs, she got on plane fine. I just always put the tabs all the way down before going up on plane and I would pretty much floor it and she got on plane fine. Sounds normal to me, maybe needs a prop change. My 342 has 22p props and gets up on plane no problem with no tabs but if you read through this forum lots of 342 guys had 24p props and struggled to get on plane until they switched to 22p props and then no problem. 

JLAS

Thanks, Liberty.  This boat, in particular, can not plane unless is at 3900 rpm (3700 rpm using tabs).Could that be because of a dirty bottom? I read in other posts that the 320 (312) with 5.0 are ok even with 10 people. Also, there is a youtube video showing how the 320 can maintain a cruising speed of 21knots at 3400 rpm (3 adults and 2 young kids).  At any rate, I am very close to pull the trigger and wanted to check in this forum. 

Liberty44140

Before pulling the trigger I would make sure the props are right. Trouble getting on plane without tabs is not that unusual for that boat but trouble staying on plane below 3900 does not sound right, I may have missed that. Definitely make sure the props are right. Also was bottom clean or nasty? On my 310 that made a big difference. @Dream_Inn has a lot of 310 experience so he may have a better view. I think the 320 is the 310 with more swim platform. 

JLAS

Thanks, Liberty for the great pointers.  Today a diver cleaned the bottom that was NOT in great condition, and actually, it needs a paint job badly.

Today's test drive was better with a clean bottom.  The boat got to plane with no problem at a top speed of 38 mph at (5000rpm) and cruised staying on plane at 3500rpm with tabs down, though @ 18-20 mph. Bellow that I don't think it would be able to be on plane. I still think with new bottom paint the boat will get to specs or closer to it. 
Thanks, once again, for your input. 

Serenity

I have a 2005 320 with 5.0 Mercs and Bravo 3 drives. I always use the tabs all the way down and 3/4 throttle to get on plane. Once on plane I bring the throttles back to 3200 and tabs all the way up unless I need to compensate for people weight. I generally have a full water tank and at least 1/2 tanks of fuel. I have been out with all the tanks full and 10 people--yes it was a bear to get on plane but it did it. I always top out at 5000rpm and 40mph per gps. Your hull and props have to be squeaky clean. I make sure I use the boat at least once a week. Even if it is a quick ride--out the canal and on plane for a mile or two. That keeps everything clean and the boat running perfect. One thing you should check is what they are carrying in all the cabinets. There may be a ton of useless items along for the ride just in case they will need it.

Dream_Inn

Sorry, late to the party (but still want a drink ).  My 310 had the 5.0 MPIs and the boat topped just over 40mph.  No paint on bottom, so that may be the small difference from what you tested.  The lower speed just staying on plane sounds about right.  That boat likes to cruise around 3800 or so and get about 30mph.

JLAS

Thanks, Dream Inn

frodo13056

Here's some pictures of my 2008 320 GPS shortly after I purchased it. The GPS pictures show you the MPH, fuel burn and MPG / GPH, etc. I also took pictures of the tach at the same speed to check RPM. Note that we carry lots of stuff since we weekend on the boat most every week during the summer (read: I have a TON of stuff on the boat) and the boat does have bottom paint.

The first picture is at 27.8 MPH at about 3600 - 3700 RPM. Note that the factory tachs leave a lot to be desired when it comes to accuracy so when reading the tachs it's a bit of a guess. 




This set of pictures is the "sweet spot" for my 320 - right around 32 MPH at 4000 RPM gets me around 1.2 MPG. 
The second picture below was at wide open throttle which got me to 37 MPH at about 4600 RPM - I have since had it up to 42 MPH at around 4900 RPM. Note the differences in fuel burn, etc. A dirty bottom (algae growth, barnacles, etc.) will impact your speed due to drag - I learned that the hard way years ago with a boat the I kept in the water with no bottom paint. I actually thought I had an engine problem because it would barely get on plane with just me on board. I pulled the boat and scrubbed the bottom and all was good. It does't take a lot of growth to adversely impact the boat performance.

reneechris14

Nice info and pics,I like that

Willhound

Cool pics Frodo.  Did you pre program the Plotter with fuel burn rates at various speeds, or is it picking up the data from the engine computer? 

JLAS

Thank you ALL and  Frodo for the great info and explanation.  I feel much better as I am pretty sure the bottom is the main cause for the slower speeds at certain RPMs.  At any rate, thank you for taking the time and the info.  I am excited about the 320.  It is a nice boat.   The more I'm in it, the more  i like it!  Next project, putting a table on the u-shape seating arrangement.