Boat doesn't rise. Prop issue?

[Bman2895]

So info.

Boat is a 1990 Cajun paul Elias edition.
18' 6" long × 88" beam. Everything i find shows its a 1,200 lb bare boat. The boat appears very dry always kept under cover as far as I can tell and doesn't leak so i don't believe theres any water logging going on. Its got 2 group 27 lead acid deep cycles. Minn kota power drive on the bow. Not a ton of gear in it. Usually runs with 2 people. Roughly 480lbs of persons. Plate rates the boat at 185hp.

Engine is an old johnson v4 mix match. Its a 1978 140hp. it was at some point rebuilt with .30 over pistons and had the lower compression heads swapped on Instead of the high compression heads the 140hp usually has. And it has 1983 140hp carbs on it(smaller bore) so no Idea what it really makes for HP. But surely not the 140hp.

Prop is an OMC 13x19 stainless.

WOT rpms according to the tach is 5000. If the old tach in the dash is to be believed anyway.

Does have a small jack plate. I think 4" and im not sure of the exact height, but the motor is on the lower holes on the engine to set it up higher and the jackplate is fully raised. It was 2 higher before running alittle lower when I got it. Changing it made no difference to my issue.

Currently the boat runs 37-39 mph depending on conditions. It planes out decently with 2 people. Planes slow with 3 people unless one sits on the deck. But top end mph remains the same regardless.

The issue I've noticed is the bow doesn't really rise/lift on plane. It gets up on plane and sits the bow down. Kinda plows? Going off the spray it looks to be riding with half the hull still in the water? The spray(wake or whatever its called) is just ahead of the console. Im used to previous boats riding just behind the seats. And it feel like its holding the engine back. And thats trimmed as far as the trim up will go (not sure if it hits a limit or if the trim can't overcome the engine pushing down against it) Throws a decent rooster tail on plane.

Can this be improved? Namely the bow riding low/"plowing". I dont expect a top speed much better than 40mph. I wouldn't think that engine could manage it but mid 40s would be nice. Could i pitch down? I don't think 5k is the max wot rpm for these. Or am I as good as ill get and limited by the the hp ive got?

 

[jimmbo]

WOT range for that motor is 4500-5500. While the motor is rated 140hp at 5000rpm, the actual rpm for peak power on the 140 is closer to 5400rpm. For an 18ft boat, I would say the 19" pitch is a little high, a 17 would be a better choice.
As for the lack of bow rise and plowing. I will assume the motor has power trim. While trimming the motor in(down) will reduce Bow Rise while getting up on plane, it will also cause Plowing at speed cause the motor is leveraging the Bow down. Trimming out(up) will allow the bow to rise while on plane.


That all said, if the hull has a Hook, that too will result in less Bow rise, and Plowing.


As for your Jackplate. Those things only become important at speeds above 50mph

 

[Chris1956]

There are at least three reasons for low or no bow rise. The first is a hook in the hull. The hook will deflect water downward at the stern, lifting it and pushing the bow down. The second reason is some sort of appliance at the stern, again deflecting water downward at the stern. Does the jackplate extend below the hull at the transom? Is there anything else at the stern that could deflect water downward? Fishing boats often have elaborate FFs and depth gauges. Do the transducers deflect the water down?

Next, take a look at the shape of the hull. Some hulls have a deep vee shape that extends all the way aft, without flattening out. These tend not to have much bow rise, when operating them. Instead, the whole boat tends to rise straight up as speed increases. Those boats will often ventilate in turns, but not in straight running.

 

[dingbat]

Have a read…..

https://www.bbcboards.net/showthread.php?t=53542

 

[Bman2895]

Well like I say I doubt the engine is making what its supposed to given the smaller carbs and different heads.

I assume the jackplate was likey on there do to a bigger outboard being on their at some point that would achieve those speeds.

I don't think its a hook hull? Id have to check again. I wanna say the hull is pretty well a V all the way and right at the back there's a step up.

And I do trim all the way up on plane. Im just not sure if the amount of trim up im getting is the correct amount. How much total trim up should I have under WOT?

 

[racerone]

What bore size is marked on the " smaller " 1983 carburetors?

 

[Chris1956]

You want to trim up until the prop ventilates and then down a bit until she bites the water. A lot of motors will only trim up a specified amount, before stopping. Sometimes this can be adjusted.