WOT less than 3k

[bud37]

Good call on getting them to step up.........

[Capn Crunch]

So mech was out this morning, replaced module - pics shows part of the problem but still not completely solved. Now achieves 3400 rpm but only hits max speed of 14 mph (via handheld). Mech is thinking wrong prop, book says 16 x 16, he says he thinks it's 16 x 13?

[tomschauer]

That boat should cruise at 24 -28 mph @ around 3600 I would think and top out at a bit over 30 at 4500ish. A lower pitch prop will bring up your rpm's but not your speed.

[Capn Crunch]

Source of the post That boat should cruise at 24 -28 mph @ around 3600 I would think and top out at a bit over 30 at 4500ish. A lower pitch prop will bring up your rpm's but not your speed.

Well now I'm totally confused. Mech took Mercruiser module (see pic in previous post) with the wire loose from the connector and replaced with a Denso module. Adjusted timing and idle again and on test run we got another 400 rpm (to 3400 max). He thought maybe throttle needed adjusting so did that, pulled spark arrestor to confirm secondaries were opening and they were... still, max @ 3400 and speed @ 14mph.

Now they are going to get a retired master mechanic to come take a look... but the owner of the shop says he thinks it's the prop - how do decipher all of the mixed messages (both them, and here) I seem to be running into?

By the way, I REALLY want to say thanks to all whom have given input, I feel a bit lost in all of this... car motor = suck bang blow, boat? Seems not quite as simple lol.

Mark

[bud37]

I don't think there are any mixed messages, it seems there are a few things going on.....does the engine now run clean and smooth and are you completely on plane now or pushing a lot of water?
If the spec is 16x16 and you have 16x 13 that would mean the rpms would be higher with less speed. If a new engine/ drive etc was put in, then you probably need the boat propped correctly for the situation as it is now. Also make sure the bottom is very clean before any speed trials.....honestly I think they will sort it out....good luck Mark

[Capn Crunch]

Source of the post I don't think there are any mixed messages, it seems there are a few things going on.....does the engine now run clean and smooth and are you completely on plane now or pushing a lot of water?
If the spec is 16x16 and you have 16x 13 that would mean the rpms would be higher with less speed. If a new engine/ drive etc was put in, then you probably need the boat propped correctly for the situation as it is now. Also make sure the bottom is very clean before any speed trials.....honestly I think they will sort it out....good luck Mark

Engine seems to run much better, but feels like it's loading up. Was on the verge of planing, but felt like it needed another 5mph or so to get there. Turns out (found out only today) it's not a new engine... an engine with undetermined experience but with new heads. Botton was a fresh coat of bottom paint and had been stored @ an rv storage site before going to have engine work. Apparently PO and same shop tried a few different pitched props before landing on the 16 x 13 as ideal for this boat.

I do believe they'll sort it out as you do, just anxious to get her up to snuff b4 it's too cold to use her, heh!

thanks for the encouraging words

Mark

[bud37]

From your new explanation, sounds like you need some more balls to get on step......once on top the revs will take off......so they will or should double check the prop, double check the timing curve and total....the way the boat is acting is as it is underpowered for some reason ( engine ), or overpropped as it cant rev up.

[waybomb]

Are they checking total timing; I <believe> a V8-22 module provide should 8 degrees base+22 degrees advance, or 30 degrees when the engine is at 3600 rpm. Maybe Viper as a manual and could look it up, both base timing and when full advance is in.

[tomschauer]

all of the above plus, with the right prop (and drive ratio if the drive was changed) it should plane at 3400, even if it takes awhile to flatten out. Did you leave it at 3400 for a minute or two? and did it start to lay down?

[km1125]

Find out the drive ratio for the trans. Then you can do some simple calcs to see where that prop should be in term of speed, with a bit of slip. If it is only a 13 pitch, then you should be jumping up on plane even if you're not maxing out on speed.

I think something is still holding the engine back. The total timing thing could be that, or you have some restriction in the fuel delivery.