Log of SV Free Spirit and ships company

The chronicles of the schooner Free Spirit and her crew, embarking on an open ended journey upon the great rolling heap. Free Spirit is currently pursuing humanitarian and commercial goals in the Dominican Republic, on the island of Hispaniola. Working under the Ocean Reach USA and Paradigm Research banners, she is serving as logistics headquarters, workshop, and development laboratory for many ongoing projects. This is the log of her journey.....

Laura
CT

Sunday, January 21, 2007

What little I have learned about small sandblasters

This seems to be the kind to use - the type that has a pressurized sand reservoir.

30 seconds work with a siphon type blaster - 9CFM - useless for all but the smallest jobs.

30 seconds work with a pressurized resivour blaster - 9CFM
I wouldn't want to do a whole boat at this rate, but excellent
for spot blasting and detail work.

First you will need a biggish compressor. 10CFM@90psi would be a bare minimum. I got one from Harbor Freight for $380. I got the pressure blaster for $80, and the siphon blaster for $10.
Make sure the compressor is well away from the blasting area or your filters will be clogged in no time. You will want a face shield and respirator as a minimum, a pressurized hood to be preferred. Or you can try what I am attempting, to build a cheap pressurized suit. If you go this route, let me know what you come up with!

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Introducing Propcalc 4.0

Use Propcalc to easily match your hull with your engine, transmission, and propeller
Put the known data in the top fields, then hit the Update button to get the answers.
Results, of course, should be verified by a Naval Architect or qualified surveyor.
Data is provided for three bladed propellers of average type
For two or four bladed props, use the modifiers shown below.

Fill out the fields as follows:

Vessel LWL (ft) = Waterline length
Vessel Disp (lbs) = Vessel displacement
(max) HP = Rated Engine Max HP
Engine RPM max = Engine RPM at Max HP
Engine RPM cruise = Desired or estimated cruise rpm
(Cruise or Max) Kts = Speed to work the calculations for
Slip = Propeller efficience. 45% is average for a displacement cruiser.
Gear ratio = 1: Gear ratio of transmission
SL Ratio Adj. = This value will be added (or subtracted, if a negative value) to the calculated S/L ratio.

Key information:

If the "hp required" is greater than the "cruse HP", you have your cruise RPM set too low for your engine parameters.
If the "hp required" is significantly less than the "cruse HP", you have your cruise RPM set too high for your engine parameters.
If the "hp required" is greater than the "Max HP", then your target speed is too high for your engine/hull parameters.
The S/L ratio is calculated automaticaly based on your input. It can be adjusted if necessary, but normally it should be left alone.
If the calculated S/L ratio exceeds S/L MAX, then the results are likely to be non-predictive. Try a lower speed requirement.
SL Ratios of 1.1 - 1.4 are typical of displacement hulls. Semiplaning or planing hulls can go higher.

Typical propeller slip values:

Sailing auxiliary, barges, etc less than 9 Kts............45%
Heavy powerboats, workboats 9 - 15 Kts....................26%
Powerboats, Lightweight Cruisers 15 - 30 Kts..............24%
High speed planing boats 30 - 45 Kts......................20%
V bottom race boats 45 - 90 Kts...........................10%

Note:

it is possible to get irrational answers by irrational input , I.E specifying excessive speed for hull type and length
Any attempt to exceed hull speed (1.34 times the square root of the waterline length in feet) with a displacement hull are likely
to fail unless the hull is extremely fine (multihull) or otherwise exceptional. In such cases, an S/L adjustment would be in order.

2 and 4 bladed props:

For two bladed propellers, multiply the diameter by 1.05, and the pitch by 1.01
For four bladed propellers, multiply the diameter by .94, and the pitch by .98

PROPCALC
		Inputs:
Vessel LWL (ft) =
Vessel Disp (lbs) =
	(max) HP =
	Engine RPM max =
	Engine RPM cruise =
	(Cruise or Max) Kts =
	Slip =
	Gear ratio = 1:
	SL Ratio Adj. =

		Solutions:
	Prop rpm max =
	Prop rpm cruise =
	Pitch =
	Diameter =
	Static Thrust =
	Cruise HP =
	Cruise HP% =
	SL Ratio =
	DL Ratio =
	SL Max =
	HP Required =