grady 232 gulfstream single 250 yamaha

[treblehook17]

I cruise at 22 knts at 4500 rpms

Too slow!

Repower to 300 maybe? Twins would require a whole new bracket.

What about changing the prop from the three blade prop to a four blade?

 

[seasick]

I cruise at 22 knts at 4500 rpms

Too slow!

Repower to 300 maybe? Twins would require a whole new bracket.

What about changing the prop from the three blade prop to a four blade?

In general, a four blade prop will have a slower top speed than a three assuming the same size and pitch.

 

[DennisG01]

22 knots is about 25 mph. It's certainly not "terrible" by any means, but I suppose it's on the slower side of the bell curve. However, are you sure you're trimming the engine for optimal efficiency? Do you run heavy (loaded up)? As Seasick mentioned, a 4-blade is more about the get-up-n-go than it is top end speed. You could play around quite a bit with props and maybe get an mph or so improvement. Certainly stepping up to a 300HP would do the trick and allow you run the same speed (or maybe slightly higher) at a lower RPM. But then you have to factor in the costs of jumping up to a bigger engine and whether it's really worth it. If you're putting hundreds and hundreds of hours on a season and typically travel long distances, it starts to make more sense. But if you're only going somewhat short distances, than I think it makes less and less sense to up the HP.

 

[Harpoon]

I have the same set up. 22kts is 45-4700 for me. I think they would run very nice on a bigger displacement 300. But I can live with anything around 20kts.

 

[Harpoon]

I'd be curious to know which holes your motor are bolted through. I have a hunch that mine may be a little low.

 

[seasick]

I'd be curious to know which holes your motor are bolted through. I have a hunch that mine may be a little low.

Before changing or thinking of changing mounting height, you need to check two things: The first is obvious, that you have the correct shaft length. The second is to see where the cavitation plate is relative to the water surface when at cruising speed and appropriately trimmed. It should be even to an inch lower than the bottom of the keel. There are some cases where the plate could be a bit higher so that when trimmed up, the cavitation plate does not act like an anchor but I don't know if that really applies to our type of vessels.

 

[suzukidave]

what model yamaha 250 and what pitch prop do you have?

if I assume a 15" prop pitch and 1.75 gear ratio you are getting 41% slip. that is pretty bad. it seems like you could do quite a bit to get that down by adjusting motor height and trying different props.

 

[seasick]

what model yamaha 250 and what pitch prop do you have?

if I assume a 15" prop pitch and 1.75 gear ratio you are getting 41% slip. that is pretty bad. it seems like you could do quite a bit to get that down by adjusting motor height and trying different props.

We need the year and model motor as well as prop specs.The gear ratio could be anywhere from 1.75 to 2.01.
We can't assume the prop pitch either.

 

[DennisG01]

what model yamaha 250 and what pitch prop do you have?

if I assume a 15" prop pitch and 1.75 gear ratio you are getting 41% slip. that is pretty bad. it seems like you could do quite a bit to get that down by adjusting motor height and trying different props.

We need the year and model motor as well as prop specs.The gear ratio could be anywhere from 1.75 to 2.01.
We can't assume the prop pitch either.

In addition, it would also be helpful to know the WOT RPM/Speed and what type of load that is with.

 

[gw204]

It should be even to an inch lower than the bottom of the keel.

Disagree. If your engine is transom mounted you want the antiventilation plate even with or slightly (1/2" or so) above the hull buttom. If bracketed, the general rule is get that plate 1" above the bottom for every 12" of setback. I ran the 200's on my bracketet Sailfish about 1-1/2" above the hull bottom and could have gone higher.

 

[suzukidave]

what model yamaha 250 and what pitch prop do you have?

if I assume a 15" prop pitch and 1.75 gear ratio you are getting 41% slip. that is pretty bad. it seems like you could do quite a bit to get that down by adjusting motor height and trying different props.

We need the year and model motor as well as prop specs.The gear ratio could be anywhere from 1.75 to 2.01.
We can't assume the prop pitch either.

i agree we need the actual info, but i took a (poorly) educated guess at a typical setup to illustrate the apparent problem. i did not know yamaha did a 2:1 ratio for the 250. if the ratio is 2:1 then it is 31% slip on a 15" prop which is still pretty high. if he is on a 13" prop and a 2:1 gearcase that gets us down to 20% and might begin to explain it.

 

[seasick]

It should be even to an inch lower than the bottom of the keel.

Disagree. If your engine is transom mounted you want the antiventilation plate even with or slightly (1/2" or so) above the hull buttom. If bracketed, the general rule is get that plate 1" above the bottom for every 12" of setback. I ran the 200's on my bracketet Sailfish about 1-1/2" above the hull bottom and could have gone higher.

Your recommendations for a bracket motor are correct. I wasn't taking that into account for my post.
I read many opinions about cavitation plate height.
There are some topics where I should leave my too cents out. This is one of them

 

[Harpoon]

I have the 2007 Yamaha F250, turning a 17 pitch stainless Yamaha prop. The engine is on a bracket. I should have taken a photo of the cav plate while running. WOT is about 5400.

 

[suzukidave]

I have the 2007 Yamaha F250, turning a 17 pitch stainless Yamaha prop. The engine is on a bracket. I should have taken a photo of the cav plate while running. WOT is about 5400.

how fast at 5400rpm?

22 knots = 25 mph @ 4500
gear ratio = 2:1
that seems like 31% slip.

 

[Harpoon]

I don't recall probably 26-28...

 

[HMBJack]

It is a heavy, wide, boat for one engine. Twin 150's would be the power of choice IMHO. I suppose you could go to a 4 blade prop (choose carefully) and take weight off the boat (you'll likely easily find 100-200 pounds). You might be satisfied with that. On my 330 Express, moving from 3 blade to 4 blade props helped my torque and lowered my RPM's at cruising speed (by ~10%) while maintaining the same cruise speed and nMPG. Just sayin...

 

[suzukidave]

yes it is a very different hull size and weight class from mine. still i would think they could be made to plane more efficiently with a resulting increase in speed even with a lower pitch prop. i would guess he needs something 15" pitch with lots of bite, but that's a guess.

 

[Harpoon]

I wouldn't say there is difficulty or delay getting on plane.

 

[suzukidave]

I wouldn't say there is difficulty or delay getting on plane.

sorry, i was talking about the fact that once you are on plane at cruising speed your prop is operating at under 70% efficiency. that seems quite low to me. i am used to seeing 85% efficiency or better, but i am also used to smaller boats with less beam and weight. for example, i would expect a 17" prop on a single 250 to propel a 22' grady to around 45mph maximum speed.

you have a larger heavier 23' boat and that together with maybe some bottom paint may explain the difference. it is just new info for me.

 

[Harpoon]

The 232 definitely displaces some water compared to most 23 footers... She's sure footed !