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03-01-2015, 03:05 PM #1Team Member
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VIDEO - Minimizing BOUNCE question for 1961 Mercury 800 FGS inline 6?
Is there a way to reduce the bouncing time when during trim up of motor?
In the video the boat settles nicely at about 46-47 mph, but while trimming the motor up, it bounces. I can make it run at 45 all day without bouncing by trimming motor in, but to get max speed, need to trim up - hence the bouncing.
I just built this boat. It's just over 14' long, planked in mahogany and maple. Motor is a freshly rebuilt 1961 Mercury 800 FGS long shaft. Prop in video is a 23" pitch aluminum, vented 3 blade made by Turning Point (Hustler).
I have a CMC trim/tilt unit on and this creates setback of about 7" or so. The hull w/out motor and dry is about 500 lbs. The cav plate is 2 3/8" above the pad.
The bottom is a shallow "V" with a pad. I spent hours making sure it was dead flat when I built it.
I need to figure out tach settings, also.
Photos of the boat:
Video:
Thanks,
PeteLast edited by peter1708; 03-01-2015 at 03:05 PM. Reason: added word "motor"
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03-01-2015, 06:54 PM #2
What a beautiful boat and motor combo! Great work man.
My Viper bounces like that between 30 and about 50, so I just either go slow or fast. I'm sure you can play with weight distribution and minimize the bouncing, but it may cost you top end speed.
Trace
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03-01-2015, 07:03 PM #3Team Member
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Hi Trace,
Thanks for looking, appreciate the compliment!
I didn't know if there was a particular technique to get past that bouncing quickly or not. I'd love to go faster with this boat, but I think I'm probably maxed out with this motor, might try to raise it some more (keeping an eye on the water pressure), etc.
Pete
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03-08-2015, 09:25 PM #4
Pete,
Beautiful boat and very nice vintage motor pushing it. My dad used to have a '62 700. The bouncing is called porpoising and is caused by too much weight in the rear. If you got trim tabs and installed them on the transom out to the chines that would fix your porpoising problem, however if the tabs were still contacting the water at WOT it could slow down your top speed a click. I do know a way to get better top speed... You are running WAY too much pitch! That motor should be spinning up 5800-6000 rpm at top speed!
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03-08-2015, 09:36 PM #5
If your trim was faster you'd probably run right through the porpoise and take a set.
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03-09-2015, 01:31 AM #6
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03-09-2015, 07:52 AM #7Team Member
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Thanks for the responses and compliments.
croSSed - you're absolutely right about the prop - I subsequently found out it had a large amount of cup put into it, I'm told 2"-3"! No wonder it would spin up.
wanagofass - Good call on the slow trimming. I found out the rectifier was bad, and although these old motors don't charge real well, it was not charging at all and the battery was almost dead in that video. Replaced the rectifier and, with a freshly charged battery, she got through that porpoising very quickly.
Capt. - Thanks for the info on the peak HP rpm.
Also changed props to a 14" x 21" 2 blade Merc bronze prop (uncupped) this weekend and just touch 50 GPS at 5600 rpm. I think I've got good data now to help narrow down choices to find a good prop.
Thanks again,
Pete
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03-09-2015, 09:23 AM #8
Beautiful boat. My first motor was a white 80 Merc. As boater and a professional cabinet maker I applaud your work. That looks good enough I wouldn't care how fast it was.
RockTeam Junk
No sparkling wiggles in here, only dump truck grinches.
"Screamin Heathen"
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03-09-2015, 11:01 AM #9Team Member
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Thanks for the compliments Rock. I chose the motor based on the design of the boat - I think they work well together performance wise and visually.
Pete
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03-10-2015, 12:02 AM #10
Here is something that you might try before going crazy trying to solve this. If it is possible to raise the motor some more without losing water pressure, try to get your hands on a lower pitch chopper prop like a 22" pitch or less. Choppers will help hold the front of the boat more than what you are running. Surface style props like a chopper have this effect when some of the upper portion of the blades are out of the water. Chopper props really do have more lifting ability thnt other props when they are run in more of a sufacing condition. Really worth a try.
Best wishes, Carter Powell
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03-10-2015, 09:06 AM #11Team Member
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Carter - Thanks for the advice!
I've read that there are two types of chopper prop, small ear and large ear. For this motor, would a small ear be best?
Pete
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03-10-2015, 09:25 AM #12Team Member
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This is a new video taken from shore (crummy camera and from a distance) showing the boat running full speed at start, then I slow down.
Prop is a two blade 14" x 21" (not cupped) bronze, 5600 rpm just touching 50 mph. Was able to rev to 5900 or so with trim, but no extra speed. Motor looks level trimmed out.
Wonder if a cupped prop two blade or a chopper as mentioned above would be beneficial? Would like to keep rpms at a reasonable range.
Anyone know what's up with the spray when I trim motor in?
Last edited by peter1708; 03-10-2015 at 09:27 AM. Reason: added comment on trim/spray
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03-10-2015, 11:57 AM #13
Pete,
As I've said before, prop availability could be an issue. Does that motor have a thru hub exhaust? And how many splines on the propshaft? If a good basic chopper in the 22" range like Carter says, you may get what you're looking for. If not, that 21" two blade sounds like it needs just enough cup to make it hook up when you trim higher, or raise the motor. Where's the water pick up on that baby? A pic of your lower unit would help these guys help you. They really know their stuff. I know the Capn. personally and he's quite knowledgable. IF you can get a chopper to fit, I'd go small ear. Really need to know about your lower unit...
Trace
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03-10-2015, 12:22 PM #14
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03-10-2015, 12:41 PM #15Team Member
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Hi Trace, thank you for the suggestion - I've started a separate thread, just to keep things a little organized. I've included photos and a detailed description of the set up.
Pete